Kepling

the discussion in the comments of this report are focusing on an issue that is not the focus of my report. the title/summary of my report has been updated by a mod to reflect the comments interpretation instead of my actual report.
what my report focuses on is "mobs teleporting back to a previously used nether portal". originally i did include the issue of "nether portals linking incorrectly when horse uses them" because i saw both issues as a single issue of "mobs locations getting messed up in relation to nether portals" but i now realise that they are two separate issues.
to avoid having multiple bugs in the same report, i edited the report and removed mention of the "nether portals linking incorrectly when horse uses them" issue. regardless of this, everybody is still focusing on that because the issue occurs in some of the provided evidence. for example; "horse doesn't appear after going through portal.mp4" and "video 2.mp4". the rest of the evidence however, is showing the "mobs teleporting back to a previously used nether portal" issue.
the "mobs teleporting back to a previously used nether portal" issue is pretty much exclusive to nintendo switch although i did get a mild recreation of it on my android phone (video 1.mp4). i speculate that this is intended behaviour to help players not lose mobs after they go through a portal (mob loads in and starts wandering, once the player loads in, the mob teleports back to the portal so that the player doesn't lose the mob) but when it's an underpowered device like the switch, this event is delayed significantly and causes the mob to seemingly disappear if the player is no longer at the portal. the "mobs teleporting back to a previously used nether portal" issue potentially happens on multiple platforms but is only a "problem" on nintendo switch in my testing.

hi @GoldenHelmet i agree that MCPE-146959 & MCPE-42310 are an issue and they do appear in the videos i provided but that is not the bug i am reporting.

my video lacks context due to the limitations of the switch's recording capabilities and an oversight on my behalf. i realised my phone can constantly record and i have made three videos showing the issue in more detail.

shows a very minor but i think very relevant teleport. this occurs a few seconds after existing the nether. the horse has wandered while the player loads into the world but then teleports back to the nether portal almost immediately.

shows a horse entering a portal but when the player follows through the portal the horse is gone. the horse is then found at another portal.
i would suggest that this is very similar to what is happening in video 1 but the horse is teleporting to a different portal to the one it just existed this time.

shows a horse teleporting to a nearby portal and immediately being transferred into the overworld.
i am quite confident in this as i previously had pressure plates that would be activated by the horse when it teleports like this but have removed them to prevent the possibility of the horse entering and exiting the portal while the player loads as you described.