Normally you cannot travel through portals in spectator gamemode.
But if you go through a portal (both Nether & Ender) and after (while not being in the portal) you change your gamemode, for example, to creative, you will immediately be teleported in the dimension of the last portal you crossed.
The game just remember the last portal you travelled, and teleport you when it can (when you quit the spectator mode)
It's an easy bug to replicate, works anywhere with any portal (even the end gateway)
Thank you for your report!
We're tracking this issue in MC-279021, so this ticket is being resolved and linked as a duplicate.
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