If you build a nether portal in the nether on the very corner of a shrunk world border (100x100 world border, etc), then the portal that is generated on the overworld side will partially spawn outside the border.

To reproduce:
1. Generate a superflat world using default preset (though should work with any presets)
2. Use /worldborder set 100
3. Use /execute in minecraft:the_nether run tp 0 60 0
4. Build a nether portal on the very corner of the world border in the nether and then enter

Observed results: Part of the nether portal is outside the border

Expected results: The nether portal is fully within the border