All blocks are obsidian except for the upper south east corner
To reproduce:
/fill ~-1 ~-1 ~-1 ~-9 ~-1 ~-9 minecraft:water
/fill ~-1 ~-1 ~-1 ~-9 ~-1 ~-9 minecraft:lava
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using "destroy" will work, but then you could not use "replace"
I guess if you need to use "replace" you can fill with air then fill lava
Can confirm in 20w49a.
Can confirm in 1.16.5.
Can confirm in 21w03a.
Can confirm in 21w06a.
Can confirm in 1.17.1.

As MCPE-100607 is tracked as a bug in Bedrock Edition, I am not sure if /fill
turning water into obsidian is intended.
Yeah, it updates the lava (which is next to water) before placing the remaining blocks; it should first replace all blocks, and only then update them.
Can confirm in 1.18.2 and 22w18a.
Can confirm in 1.19.
Can confirm in 1.19.2.
The fact it creates obsidian is most likely intended - the bug is that it misses the south east corner. If you do want to properly replace water with lava, add "destroy" at the end of the /fill command