/tp <target player> @e breaks after a while

I was trying to emulate a menu of sorts in my adventure map using named armor stands as placeholders for players, and long story short eventually found that using the @e selector or anything derived from it as the <destination player> argument for /tp produced an error stating "The entity UUID provided is in an invalid format" after working a few times, unless the [target player] argument was omitted, in which case it worked perfectly well. This was with some light modding (Optifine, Schematica, some HUD mods), so I re-tried in vanilla MC and the same thing happened in a different world. Quitting and rejoining either world did not fix it, nor did restarting the launcher.