The bug
The problem that blocks remain depending on the order of updating blocks of the fill command.
(Old name: /fill lava bug
Old description: When replacing water, lava and obsidian with lava with /fill, not all can be replaced.
I checked this bug in 1.12.2 and 1.17.1.)

How to reproduce

1. Create a space of 2 by 2 or more(Height is 1 or more)
2. Fill the space with lava (r1.png)
3. Put a block of water in the lava (r2.png)
4. Replace all with lava with /fill (r3.png, r4.png)