The /stopsound doesn't correcty output it's succescount. When the specific sound isn't playing, the command still gives a positive succescount. It should only be that way if the sound was actually playing.

How to reproduce:
1. place a repeating command block with the following command:
/stopsound @p hostile minecraft:entity.endermen.teleport
2. make sure the command block is always active and press 'done'
3. make sure there are no enderman around you.
4. notice that in 'previous output' is written: [...] Stopped sound '...' for ...

The succescount should only be true when the sound was actually playing.