Tameable animals don't spawn either naturally, via the summon command or via spawn eggs in forge version 47.1.30. Notably it seems this issue does not occur in a dev environment. Here's a log:

https://pastebin.com/ztBZikuh

So ultimately this occurs due to Forge's changes regarding EntitySizeEvent. There's not a lot we can do about this short of making some significant changes, and this is also in the window where Lex Forge is known to be fairly broken.

47.1.6 Fix entity size event not being fired, changed it to split eye height and size calculations. (#9535) is the version and change log that breaks animal spawning/summoning

@Notenoughmail had nailed down the exact maximum supported version in discord.