The snow is misplaced - villagers can walk on it, go over the fence and they'll get trapped.
Fix
Either remove the snow or make villagers able to use fence gates.
MC-219614
Villagers become stuck inside snowy_animal_pen_1 because of snow that is placed too high
Open
8
(Unassigned)
Confirmed
(Unassigned)
1.16.5, 21w10a, 21w11a, 21w14a, 21w16a, 21w17a, 21w19a, 1.17 Pre-release 1, 1.17.1, 1.21.4
Retrieved
Can confirm