I've seen a lot of reports about this, however this was supposedly fixed in 1.17.1 and it wasn't.
Starting off, all of these issues and tests are on vanilla Minecraft- a fresh install and everything. (I have modded it in the past but I did a fresh install to test if it was maybe a mod as we initially noticed the issue on a modded server; it wasn't Optifine as we use Origins on the server as well. It is not a mod as all of these tests are on a freshly installed vanilla version of Minecraft and Windows 10, both my own and two of my friends' versions of Minecraft).

I currently have 3 locations in one survival world (World Neo), 2 locations in another survival world (World Akari) and 5 in a final creative world (World Koi), all holding axolotls. I intentionally placed three of them (one in each) over a few chunk borders as I know this was an issue in Bedrock as well. I placed signs outside of each with the number of axolotls in each pool as a tracker of where I started. I tracked the numbers over a span of two in-game months to see exactly where they landed and what was exactly going on.

There were at least two in each pool nametagged and leashed, no way to climb up onto land (surrounding walls of two+ blocks high and fencing on top of the wall; no way out for the critters. Felt bad for them but it was for the greater good). I also made sure that there was no way for them to escape through the bottom of the enclosures; I covered the bottom of the enclosures with resources such as stone and iron blocks. By the end, only a few of them remained in each world.

All have been fed tropical fish, bred, bucketed and then released into these areas closed off from any other exit source as all of these were made by hand. I counted them weekly in-game to keep track of the numbers as well; I would pick them all up with buckets, put them in a chest, write the number down and then release them back into the water.

Start/End

World Neo:
Pool 1: 4 : 1
Pool 2: 7 : 3
River 1: 15 : 4 (one nametagged axolotl died due to a Drowned after a Creeper blew up a fence)

World Akari:
Pool 1: 6 : 2 (one being a blue one)
River 1: 22 : 8 (this one was more of a breeding overflow as I was trying to get the blue one)

World Koi:
Pool 1: 8 : 3
Pool 2: 5 : 1
Pool 3: 2 : 0
River 1: 17 : 3
River 2: 28 : 15

I have also noticed three nametagged parrots and four cats disappearing but I haven't tracked them yet. I know this was mentioned in bugs such as MC-232558, however that was marked as invalid due to the mod, and MC-228858, but that has been reported as fixed whereas this has not been fixed.

I don't exactly know what the limit on launcher logs or what is as this test ran over the span of two weeks in real life nor what to screenshot (the signs? The pools? What do I screenshot in this case?) but this is the case I'm dealing with.

I know it's going to be marked as a duplicate and whatnot but wanted to at least bring it to your attention.