While defining a prefix-list in 1.4, user can set an ipv6 prefix, when actually this should be done in prefix-list6.
And since input is an ipv6 prefix, FRR set this prefix list as an ipv6 prefix list.

Example:

vyos@vyos# run show config comm | grep FOO
set policy prefix-list FOO-4 rule 10 action 'permit'
set policy prefix-list FOO-4 rule 10 prefix '2001:db8::0/64'
[edit]
vyos@vyos# sudo vtysh -c "show run" | grep FOO
ipv6 prefix-list FOO-4 seq 10 permit 2001:db8::/64

Using an ipv6 prefix on prefix-list should be denied, as it is in 1.3 version