Yes I found it very weird too. Anyways, after posting this, I got curious to read the output of the Pi Hole debug, in hope to find some clue.
First I found a link inside the logs about ULA ipv6 address. So I open my router config and found that config, and turned it from disable to automatic.
Them I keep going through Pi Hole config and found all that weird addresses in the DHCP leases table. A bit of searching and I found how to clear that leases. This solved the problem.
Now it looks fine:
Server: 192.168.100.2
Address: 192.168.100.2#53
Name: aspireone.local
Address: 192.168.100.2
> pi.hole
Server: 192.168.100.2
Address: 192.168.100.2#53
Name: pi.hole
Address: 192.168.100.2
Name: pi.hole
Address: fdec:4d47:b3ab:b100:f746:bc86:40d9:81f9
> exit
fellipec@aspireone:~$ ip -6 address show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 state UNKNOWN qlen 1000
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
inet6 2001:1284:f016:6a68:cf5c:dd0b:ceb9:6026/64 scope global dynamic mngtmpaddr noprefixroute
valid_lft 86113sec preferred_lft 86113sec
inet6 2001:1284:f013:e6c8:d960:a8b1:3be:c2fd/64 scope global dynamic mngtmpaddr noprefixroute
valid_lft 81662sec preferred_lft 81662sec
inet6 fdec:4d47:b3ab:b100:f746:bc86:40d9:81f9/64 scope global dynamic mngtmpaddr noprefixroute
valid_lft 6913sec preferred_lft 3313sec
inet6 fe80::dbc4:eb5e:e594:7e46/64 scope link
valid_lft forever preferred_lft forever
Unfortunatelly IPv6 is still a confusing topic to me. Thanks a lot for your help.