First Video: Changing the game mode from Creative to Spectator while inside a Nether portal, then exiting and switching back from Spectator to Creative, teleports you to the last coordinates (scaled by a factor of 8) and dimension where you previously changed the game mode. This process generates a Nether portal in the new dimension.
Second Video: Spectating a player as they travel through a portal causes a glitch that traps you inside the portal. The only ways to escape are by using /tp, changing your game mode, or sneaking. However, the latter two options teleport you to the player's exact coordinates in your current dimension, rather than the dimension they traveled to, as if you are still spectating the player only by coordinates, until you leave Spectator mode.
These mechanics apply to both the Nether and End dimensions and work in both directions.
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We're tracking this issue in MC-279021, so this ticket is being resolved and linked as a duplicate.
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