The browser may request a time frame that is still in the future for Pi-hole.
Check if the machine browsing the UI would use a different time than the machine hosting your Pi-hole.
Unrelated to your observation, I've noticed in your debug log that you have enabled Pi-hole's Conditional Fowarding.
Generally, that's not necessary if Pi-hole is acting as your DHCP server.
Consider disabling it - if you keep Pi-hole as DHCP server.
Your debug log also suggests that your router is of a FritzBox variety, and those routers support configuring them to tell clients to use your Pi-hole for DNS.
In your case, however, your router is still advertising itself as local IPv6 DNS server.
While you should disable that IPv6 option (see e.g. Unresolved ipv6 adress in my top list - #4 by Bucking_Horn), you may want to consider shifting DHCP server duties back to your FritzBox (in which case you'd want Conditional Forwarding to stay enabled).