At the border, right next to it, there starts to be a pattern between detector rails that activate when a minecart is on it, and those that don't. First, the block right next to the border is where a detector rail can't be powered, then there are 2 that can, and 2 that cannot, and that continues the pattern, I'm not sure when it stops, but it's not bugged at spawn. Disclaimer: I only tried on the positive x border, and haven't tried on anything other than 20w19a Java. (Applies on servers also, thats where i found the bug)
Steps to recreate:
1. Go to a new world in creative
2. use the command: /tp (username) 29999983 ~ 70 ~ 0 (I don't think the z value counts)
3. Place a minecart directly in front of you, on top of a detector rail.
4. Dig down 2 blocks and place a slab at the bottom so you can see the state of the detector rail. (Doesn't apply if you're using glass, because you can see through glass)
5. Check if it doesn't power the rail (if this was not bugged, it should)
6. Continue placing them in the same pattern that I did in the screenshot below