devtrackers.gg
So, I found the issue.
The Pastry Chef (-50% bakery maintenance cost) does not allow the citizens to eat bread when used in the Union.
Buggy McBugFace
I've quickly tested this on my own save and can't reproduce this. Equipping this item has no effect on the population.
If unequipping the item really does solve the problem for you, I'd ask you to get in touch with support, ideally with a video showing the issue and your savegame for further investigation.