some posts not going to single.php

I have a strange problem where some posts will open in single.php and some will simply show the index.php again.

I’m mildly (yet not 100%) sure that they all worked earlier.

If I go into the admin and do preview post it will already either go to index.php or single.php. Same posts always work and don’t work so it’s (thankfully) not a random thing.

Anyone had this before? And solved it? Any help would be greatly appreciated as I have no clue what’s causing this.