The diamond pickaxe had Fortune I and Efficiency III as enchantments. While I was mining obsidian I switched to my water bucket to turn lava into obsidian and switched to my diamond pickaxe immediatly, after I collected the water again before it could even spread.

During this process I was sneaking against the obsidian block I mined to catch it for sure before it reaches lava which was sometimes underneath it (it was an underground lava lake which I turned into obsidian maybe approx. 1 and a half in-game hours before). The ground I was standing on is Bedrock. It´s a singleplayer world. And I began this process before and it happened at the 15th obsidian block I mined.

It could´ve been the lava which was underneath which made it dissapair, because I believe I pressed Q and dropped the pickaxe. However, I´m not sure and I ended up with another water bucked in my hand instead of my pickaxe. And I´m not sure anymore if I got the obsidian block or not.

TLDR: I was sneaking against the obsidian I mined, used an enchanted diamond pickaxe with light-yellow-greenish durability left, used a water bucket to turn lava which was underneath the obsidian I mined into more obsidian and I switched between my water bucket and my diamond pickaxe really fast and didn´t pay a lot of attention. I am in singleplayer.
My coordinates were: -1080 5 129

New discovery: My diamond pickaxe was apparently only invisible in my hotbar and just reappared right now while writing this while I wanted to put my iron pickaxe into that slot. My diamond pickaxe acted like I just picked it up so it went onto the next free slot on my hotbar. And onbe of the water buckets just dissapaired as I opened my inventory. I mean the one rightmost in my hotbar. The diamond pickaxe did not dissapair.
I know, it´s confusing but I´m only describing what happens right now as I´m writing this and having Minecraft still open.

Note to the third and second screenshot: The second one is with the debug menu without visible chunk borders enabled
and the third is the one with the debug menu and with visible chunk borders enabled.