Repro steps:

1. Create an end portal frame with frame blocks facing the correct ('outward', as in standing inside and looking out of the portal) way.
2. Create an adjacent portal to east (Positive X), but place the frame blocks the opposite ('inward') way.
3. Place eyes of ender into the 'wrong' portal frame.
4. Place eye of ender to any frame block of the left row (towards negative Z) of the 'correct' portal frame.

The portal should then open next to the portal frame rather than inside it in direction of negative Z. The portal does open inside the frame when the 'correct' frame is completed. That effectively fires up two end portals.

The same works for positive Z, but instead of placing the eye of ender on the left side, place it on the right side.