We do not have yet established that different versions of dnsmasq are causing your issue.
And indeed my findings do not suggest that this is the case.
As promised, I have reconstructed your situation in my network by disabling IPv6 support as well as stopping DHCPv6 and distribution of a ULA prefix.
I can confirm that this will prompt the behaviour you are seeing on your network.
As expected, Pi-hole won't be able to hand out a DHCPv6 address range in absence of any IPv6 prefix, resulting in the same log message you are seeing as well:
dnsmasq-dhcp[1749]: no address range available for DHCPv6 request via wlan0
I can also confirm that Pi-hole v4 and Beta 5.0 exhibit the same, identical behaviour in this regard, as demonstrated by the corresponding logs:
router provides no ULA prefix, IPv6 disabled
Pi-hole is prepared to advertise its features via eth0, but unaware of an IPv6 prefix and fails to construct a DHCPv6 address range, both for v4 and Beta 5.0. Note that Pi-hole also tries to mitigate this by actively probing for a router (RTR-SOLICIT), yet still receives no prefix.
Pi-hole v4
dnsmasq[27212]: started, version pi-hole-2.80 cachesize 10000
dnsmasq[27212]: compile time options: IPv6 GNU-getopt no-DBus no-i18n no-IDN DHCP DHCPv6 no-Lua TFTP no-conntrack ipset auth DNSSEC loop-detect inotify dumpfile
dnsmasq-dhcp[27212]: DHCP, IP range 192.168.0.201 -- 192.168.0.240, lease time 1d
dnsmasq-dhcp[27212]: DHCPv6, IP range ::100 -- ::1ff, lease time 1d, template for eth0
dnsmasq-dhcp[27212]: DHCPv4-derived IPv6 names on eth0
dnsmasq-dhcp[27212]: router advertisement on eth0
dnsmasq[27212]: using local addresses only for domain use-application-dns.net
dnsmasq-dhcp[27212]: no address range available for DHCPv6 request via eth0
dnsmasq-dhcp[27212]: RTR-SOLICIT(eth0) b8:27:ef:ba:de:da
dnsmasq-dhcp[27212]: no address range available for DHCPv6 request via eth0
Pi-hole Beta 5.0
dnsmasq[1749]: started, version pi-hole-2.81 cachesize 10000
dnsmasq[1749]: compile time options: IPv6 GNU-getopt no-DBus no-UBus no-i18n no-IDN DHCP DHCPv6 no-Lua TFTP no-conntrack ipset auth DNSSEC loop-detect inotify dumpfile
dnsmasq-dhcp[1749]: DHCP, IP range 192.168.0.241 -- 192.168.0.251, lease time 1d
dnsmasq-dhcp[1749]: DHCPv6, IP range ::200 -- ::2ff, lease time 1d, template for wlan0
dnsmasq-dhcp[1749]: DHCPv4-derived IPv6 names on wlan0
dnsmasq-dhcp[1749]: router advertisement on wlan0
dnsmasq-dhcp[1749]: IPv6 router advertisement enabled
dnsmasq[1749]: using only locally-known addresses for domain use-application-dns.net
dnsmasq-dhcp[1749]: no address range available for DHCPv6 request via wlan0
dnsmasq-dhcp[1749]: RTR-SOLICIT(wlan0) b8:27:ef:ba:de:da
dnsmasq-dhcp[1749]: no address range available for DHCPv6 request via wlan0
router provides ULA prefix
Pi-hole is aware of fd00:: prefix and constructs its DHCPv6 range accordingly, both for v4 and Beta 5.0.
Pi-hole v4
dnsmasq[26288]: started, version pi-hole-2.80 cachesize 10000
dnsmasq[26288]: compile time options: IPv6 GNU-getopt no-DBus no-i18n no-IDN DHCP DHCPv6 no-Lua TFTP no-conntrack ipset auth DNSSEC loop-detect inotify dumpfile
dnsmasq-dhcp[26288]: DHCP, IP range 192.168.0.201 -- 192.168.0.240, lease time 1d
dnsmasq-dhcp[26288]: DHCPv6, IP range ::100 -- ::1ff, lease time 1d, template for eth0
dnsmasq-dhcp[26288]: DHCPv4-derived IPv6 names on eth0
dnsmasq-dhcp[26288]: router advertisement on eth0
dnsmasq-dhcp[26288]: DHCPv6, IP range fd00::100 -- fd00::1ff, lease time 1d, constructed for eth0
dnsmasq-dhcp[26288]: DHCPv4-derived IPv6 names on fd00::, constructed for eth0
dnsmasq-dhcp[26288]: router advertisement on fd00::, constructed for eth0
dnsmasq-dhcp[26288]: RTR-ADVERT(eth0) fd00::
dnsmasq[26288]: using local addresses only for domain use-application-dns.net
Pi-hole Beta 5.0
dnsmasq[1958]: started, version pi-hole-2.81 cachesize 10000
dnsmasq[1958]: compile time options: IPv6 GNU-getopt no-DBus no-UBus no-i18n no-IDN DHCP DHCPv6 no-Lua TFTP no-conntrack ipset auth DNSSEC loop-detect inotify dumpfile
dnsmasq-dhcp[1958]: DHCP, IP range 192.168.0.241 -- 192.168.0.251, lease time 1d
dnsmasq-dhcp[1958]: DHCPv6, IP range ::200 -- ::2ff, lease time 1d, template for wlan0
dnsmasq-dhcp[1958]: DHCPv4-derived IPv6 names on wlan0
dnsmasq-dhcp[1958]: router advertisement on wlan0
dnsmasq-dhcp[1958]: DHCPv6, IP range fd00::200 -- fd00::2ff, lease time 1d, constructed for wlan0
dnsmasq-dhcp[1958]: DHCPv4-derived IPv6 names on fd00::, constructed for wlan0
dnsmasq-dhcp[1958]: router advertisement on fd00::, constructed for wlan0
dnsmasq-dhcp[1958]: RTR-ADVERT(wlan0) fd00::
dnsmasq[1958]: using only locally-known addresses for domain use-application-dns.net
As stated before, let me also emphasize that this behaviour is to be expected:
So your observation indicates that your Pi-hole is unable to solicit an IPv6 prefix from your network.
That observation is fully supported by my test results, with consistent behaviour for Pi-hole v4 as well as Beta 5.0.
This leaves us with two potential root causes for your problem:
a) your router has stopped distributing an IPv6 prefix
b) something prevents Pi-hole from receiving an IPv6 prefix
As you have excluded your router's recent firmware update from being a cause, I would recommend to take a closer look at you router's IPv6 settings for addressing a).
As for b):
RAs are only visible to machines in the same network segment.
Did you perhaps recently change something in your network infrastructure, like introducing a new modem or an additional router, access point or switch? Or did you perhaps relocate your Pi-hole machine so it now uses a different connection than before?