When swimming in lava, the player can activate their elytra. This is not the case when swimming in water.
To reproduce
Create a large pool of lava
Equip elytra and fire work rockets
If in Survival, drink a potion of fire resistance
Jump into the pool
Press the Jump key (space by default) when submerged in the lava
Use your firework rockets
You can also try the same with a water pool, in which case the elytra won't activate and the firework rockets cannot be used.
Expected behaviour
This behaviour should be the same for water and lava. So either of the following would be expected to happen:
The elytra would not activate while submerged in lava, just as is the case when swimming in water right now
It should also be possible to enable elytra while submerged in water.
Observed behaviour
The elytra can be activated when submerged in lava, but not while in water.
Related issues
is duplicated by
Attachments
Comments

Dupe of MC-97190

I didn't mean that. I meant you can start flying from the swimming state.

Please unremove it.

This is not a duplicate. MC-97190 is talking about gliding into lava. This is talking about activating elytra in lava. Please mark as unresolved.
I've reopened the bug report as it indeed seems like a separate problem.

Can confirm in 1.21.1 Hotfix.