The bug
In creative mode filling a cauldron from a water or lava bucket gives the player an empty bucket, provided they don't already have one in their inventory. This didn't happen in 1.16.5 so it's introduced in 1.17's first snapshot.
Video demonstrating the issue:
[media]How to reproduce
Place a cauldron
Get a water bucket
In creative mode, right click the cauldron with the water bucket
→❌ Notice an empty bucket is added to the hotbar
Related issues
is duplicated by
relates to
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That is an intentional feature, added in 1.16, please read the changelog for more information.

That is not true, in 1.16.4, you don’t get an empty bucket in creative mode when putting water into a cauldron.
Can confirm that this didn't happen in 1.16.4. Reopened.
Might still be intentional though – after filling a cauldron you might want to empty it again, and you need an empty bucket for that.

But you don't want your creative inventory full of buckets do you?
If you want to empty it, just get a bucket yourself!
You only get one bucket in total, if you already have one, it won't give you another one.

I feel like it's inconsistent with how buckets normally work though - you don't get an empty bucket when you place water in creative mode, why should the cauldron be different?
Can confirm in 20w51a.
Can confirm in 21w03a.

I feel like this is a Won't Fix since the MCPE version of this bug (MCPE-13798) was resolved that way so I would assume the Java Edition version (this one) would go the same way.

I also tested 1.16.5, in which this issue does not happen like in 1.16.4.
Can confirm in 21w13a.

But wasn't that a feature introduced in 1.15 or 1.16 together with getting filled food bowls from mooshrooms in creative. I thought that was WAI...
Can confirm in 21w14a.
Can confirm in 21w16a.

This behavior is not listed in the changelog for 1.16 pre-release 1 (https://www.minecraft.net/en-us/article/minecraft-1-16-pre-release-1), if that's what you're talking about. The changelog talks about a different change:
"When you use an empty bucket on water in creative mode, you now get a bucket of water. If you have a bucket of water in your inventory already, no additional water buckets will be added when you use your empty bucket on water."
The change described here was the fix for MC-9856.
Can confirm in 1.18.1.

Confirmed in 1.18.2
Doesn't happend if you already have an empty bucket in your inventory or if your inventory is full.
Can confirm in 1.19.
Can confirm in 1.19.2.

Still in 1.19.4. I think this is a worthwhile fix to creative mode, as you can just break and place the cauldron again if you want to change its contents.

Can confirm from 1.19.4 to 23w13a.

Affects 1.20.6

Confirmed in 1.21.3

It seems to still happen in 1.21.4 and with water bottles as well! This makes me believe the issue is with the interaction itself, and makes this the same issue as MC-269237