The PortalCreateEvent does not return the correct amount of blocks. It only returns the portal blocks and a single obsidian left and right (See picture). In version 1.15.2 and 1.14.4 more blocks of the frame are present in the list, but still not all (See second picture). I didn't test it in 1.13.2, since the methode changed from 1.13.2 to 1.14.4.
Going from the description of the event, the old behavior and the behavior of the reason nether pair. getBlocks should return all obsidian blocks of the frame.
The provided test plugin prints the amount of blocks in the list and the reason to the console. It also places the blocks which are present in the list 10 blocks higher in the air. (orange wool for obsidian and purple for nether portal). The source of the test plugin can be seen here.