My pen animals won't graze after clearing snow, and are starving

[Zorkk]

I can't find anything online about this being some kind of intended feature, so I'm assuming it's a bug.

There are plenty of uneaten bits of grass, bushes and nettles, though I have to clear the snow to reveal the grass. It's 3 degrees outside, 14th of Decembary. The pen marker says nutrition growth is 29.61 and nutrition consumption is 18.83. I have chickens, turkeys, goats, muffalos and boomrats in the pen. They all graze perfectly happily 99% of the time. For some reason, the boomrats all started starving before any of my other animals. They're my only animals that don't have to live in a pen. This is the second time in a few in-game days this has happened. Last time, I had to leave simple meals in their pen for them to eat, and at some point they simply went back to grazing like they usually do. Both times this has occurred are when it has snowed.

Thanks in advance.

[Doomster]

Most probably, you've bumped into an insidious game mechanics on leafless plants. It's described in wiki: https://rimworldwiki.com/wiki/Plants#Temperature