Multiple Nether-side portal frames created by same overworld-side portal

In case this helps: Some of the time from the Nether side, but never from the Overworld, the portal has only 2 purple blocks (in the bottom row) instead of the full 6. In this state, it still takes me back to the Overworld. The portal is only disabled when no purple blocks at all are present, which seems to happen only when adjacent netherrack is on fire.

Destroyed portals don't get reused, this is working as intended.
The partly destroyed portal issue is a duplicate of MC-30897