In 1.21 Pre-Release 4 primed TNT that traveled through a portal should no longer be able to destroy nether portal blocks. However, when the world is closed and reloaded after the primed TNT traveled through the portal (but before exploding), the TNT will destroy the portal blocks.

How to reproduce:

1. Build a Nether portal
2. Place a TNT next to the nether portal
3. Light the TNT (it might need a few tries for the TNT to move in the direction of the portal, placing a few blocks around the TNT might help)
4. Close and reopen the world
5. Travel through the portal yourself
6. See the TNT destroy the nether portal blocks

Cause

The primed TNT doesn't save the usedPortal flag to NBT, causing it to get lost when reloading.