Steps to Reproduce

1. Create a new world in creative
2. Build a new portal here: /execute in minecraft:overworld run tp @s 11994.46 68.00 -5352.42 -267.55 71.93
3. Go through and build a portal here on the nether side: /execute in minecraft:the_nether run tp @s 1500.31 62.00 -671.71 92.45 71.93
4. Go back through
-> you will get the advancement
5. /advancement revoke @s only minecraft:nether/fast_travel
6. /gamemode survival
7. Go through both ways again
-> you will get the advancement once more

What I expected to happen

Only the "we need to go deeper" advancement triggering

What actually happened

For this portal pair, in two different worlds, I got the advancement "Subspace Bubble" without travelling at all. Might be related to how far from the origin these portals are. I could not reproduce using a differnt portal pair. Not sure what exactly this is related to.