It appears the functionality that provides the 'never forget chunk data' is intercepting the unload and preventing it from firing. However, since this isn't a 'feature' in the fabric build, it doesn't look like I can disable it. And, by disable, I mean not intercept the call (it's interfering with another mod that also triggers on chunk unloading).

Thank you for pointing this out. I added fix in 0.13.2, now fabric version will never skip UnloadChunkS2CPacket.

This resolved it, thanks!