I could confirm the same issue for Bedrock Windows 10 version 1.16.220 with a portal used many times without any problems until today (in the world played for the last 3-4 months - most probably it was created with version 1.16.201). I entered the portal in the overworld at coordinates 335,82,-95. The portal is normally linked to the one in Nether at coordinates around 29,71,-12 and was working reliably until now. However, this time I appeared in Nether at coordinates 335,87,-95 (i.e. more-less the same coordinates as its overworld counterpart).
The minor issue (besides the troubles with the incorrect position of the portal) was that the newly created portal in the nether (the unexpected one) - was not shown. The portal was probably there since the block of obsidian appeared after an attempt to place another block at the position where the portal was located. After the reload of the game whole portal was shown correctly.
I entered the portal a few minutes after the game was started and there were quite unpleasant lags - the game was slowed down few times for few seconds with stopped graphics.Even the transfer from the overworld to the nether took an unusually long time. Thus maybe it could be indeed related to some autosave as discussed above.
Extra notes: The issue described above appeared shortly after the game was started. After the start of MC there was shown a message on an update to 1.16.220. However, the same issue happened to another player in the same world mentioned above (accessed via LAN multiplayer from Bedrock for Android version most probably still v. 1.16.210) a few hours before the update at host game at Win 10 was loaded. In this case, it was much less convenient - the player entered the overworld roughly at coordinates 1544,77,-670 and was thrown in the nether to -1930, 107,327. After the next use of the portal, the player appeared at skyblock with x coordinated around 15500. However, also in this case it happened few minutes after the game was started.
Smyon Pa
I could confirm the same issue for Bedrock Windows 10 version 1.16.220 with a portal used many times without any problems until today (in the world played for the last 3-4 months - most probably it was created with version 1.16.201). I entered the portal in the overworld at coordinates 335,82,-95. The portal is normally linked to the one in Nether at coordinates around 29,71,-12 and was working reliably until now. However, this time I appeared in Nether at coordinates 335,87,-95 (i.e. more-less the same coordinates as its overworld counterpart).
The minor issue (besides the troubles with the incorrect position of the portal) was that the newly created portal in the nether (the unexpected one) - was not shown. The portal was probably there since the block of obsidian appeared after an attempt to place another block at the position where the portal was located. After the reload of the game whole portal was shown correctly.
I entered the portal a few minutes after the game was started and there were quite unpleasant lags - the game was slowed down few times for few seconds with stopped graphics. Even the transfer from the overworld to the nether took an unusually long time. Thus maybe it could be indeed related to some autosave as discussed above.
Extra notes: The issue described above appeared shortly after the game was started. After the start of MC there was shown a message on an update to 1.16.220. However, the same issue happened to another player in the same world mentioned above (accessed via LAN multiplayer from Bedrock for Android version most probably still v. 1.16.210) a few hours before the update at host game at Win 10 was loaded. In this case, it was much less convenient - the player entered the overworld roughly at coordinates 1544,77,-670 and was thrown in the nether to -1930, 107,327. After the next use of the portal, the player appeared at skyblock with x coordinated around 15500. However, also in this case it happened few minutes after the game was started.