When emptying a bucket containing water or lava, the contents will be placed, but the bucket disappears from the inventory.
When picking up water or lava, the bucket will not disappear if it was in a stack when picking up, but does disappear if it is a single bucket.
Linked issues
Comments

Thank you for your report!
We're tracking this issue in MC-275757, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as Fixed. Please check the Fix Version/s field in that ticket to see in which version this behavior was or will be fixed.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support (Technical Issues) – 📧 Microsoft Support (Account Issues)
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki
Duplicates MC-275757 which has been fixed in a future update 🙂