This issue is present in Create 1.16.4+ 0.3.1c
I am using Minecraft 1.16.5 and Forge 36.1.4
This creates the issue where you can convert certain containers into buckets, or turn containers into other types of containers.

This image is a screenshot of JEI, where a frozenup:mug_of_milk is filled with create:chocolate to make a create:chocolate_bucket. This of course deletes the contents of the mug and turns it into an iron bucket.

The mods I've observed this issue with are:
Frozen Up (CurseForge link)
Affected items: frozenup:mug_of_milk, frozenup:mug_of_chocolate_milk, frozenup:mug_of_truffle_hot_chocolate
Mystery Milk (CurseForge link)
Affected items: All buckets of milk, specifically mysterymilk:spider_milk, mysterymilk:phantom_milk, mysterymilk:slime_milk, mysterymilk:ravager_milk, mysterymilk:squid_milk

Having same issue with Edelwood buckets from Forbidden Arcanus. Contents or not, it'll replace the bucket with an iron bucket and the input fluid from the filler. I've tried placing empty files at the designated recipe ID as well as removing through KubeJS to no effect.