By playing today Minecraft on Snapshot 20w07a I found out, that if you use a bucket holding a shield in your second hand, you'll be unable to activate it (shield) by holding correct key, even if you change item. 

You need to switch shield with 'F' key in order to restore funcionally.
I'm adding a videoclip which will show better this issue.
I hope it'll be repaired soon

Marcin