When looking at a block while underwater, there is no hovering border/hitbox around the block. There appears to still be a hitbox while in lava on the other hand.
This issue arose before 1.12.2, although I cannot report a bug for a version that far back. I found one other issue outlining this bug but it was flagged as a duplicate, even though the report it "duplicated" does not outline this issue. See MC-117513.
https://youtu.be/HUQzx1-T2GQ