As the bug (MC-90969) from long ago seems to be back:
Stack size doesn't matter, right clicking a cow with a bucket or a mooshroom with a bucket or bowl will not give you milk, mushroom stew or suspicious stew while in creative mode. It works perfectly fine in survival.
Linked issues
duplicates 1
Comments 4
This was always the case. The resolution of MC-90969 is "Works As Intended."
In that case, I might have to submit a suggestion for change of creative-use-item behaviors to the feedback page, as that is nor really intuitive. I guess it aligns with water and lava buckets not filling/emptying when used in creative, but as that is something slightly different, (which can also be annoying at times), maybe there should be a reconsideration of how things are intended to be used. You should be able to milk cows, receiving an additional bucket/stew and for empty buckets in water, an additional water bucket should appear. That would be more useful.
Also, you can milk cows/get liquids in creative in bedrock edition, resulting in exactly the behavior that I'd like for Java. Soo... if Mojang is porting features from bedrock to Java now (slowly but surely) this WILL come as intentions change...
Normally, I am all in favour of Java-"features" vs. bedrock, but in that case, Microsoft/the bedrock devs seemingly had a better view on what is of good utility.
I can confirm this issue.