The following situation:
I am currently in the spawn chunks where I have some redstone set up. In a different part of the world I have entities which are being tracked inside a scoreboard. Now in the spawn chunks there's a command block being triggered by a clock which selects the entities thru their scoreboard and makes them execute a command (i.e. particles). When I now teleport myself to where the entities are (previously unloaded because not in the spawn chunks) the chunks get loaded (of course) but the entities don't run any commands. Relogging into the world solves the issue.

To summarize:
Selecting entities based on their scoreboard values works fine. But if I teleport away (unloading the chunks) and then teleport back to the entities (reloading the chunks) the entities don't get selected anymore.

This might be a problem with loading/ unloading scoreboard values for the entities but that's just a guess.

Additional confusion:

• For some reason this doesn't affect all entities.
• I was not able to recreate the issue in a different world.

I would love to hear if anybody else was able to recreate this in some form. I know a bug which can't be recreated is hard to fix and I could totally understand if you didn't fix it.

If you would like additional information or a world download so that you can check out the issue yourself just tell me.