As of Requiem 1.5.0, Pandemonium 1.5.0, and Minecraft 1.16.4, when you change dimensions, you lose your flight capabilities until you possess something new.

To reproduce:

1. Be in spirit form.
2. Start creative flight.
3. Change dimensions.
4. You lose your creative flight capability.

Notice that I can not reproduce this using Nether Portals as of Requiem/Pandemonium 1.7.6, but I can reproduce using commands.
To reproduce:

1. Create a new world with cheats enabled.
2. Get yourself in spirit form.
3. /execute in the_nether run tp @p ~ ~ ~ (use the command, not a Nether Portal)
4. /execute in overworld run tp @p ~ ~ ~ (use the command, not a Nether Portal)
5. Try to start creative flight.
6. Notice that you can't use creative flight.
7. Possess something, then go back into spirit form.
8. Notice that you now have creative flight.

Cannot seem to reproduce. What type of spirit were you in?

Seems to work fine in the latest version, feel free to reopen with more details if it still happens.