@KN4THX I have a 3B so alas, I can't tell you anything about the 3B+.
There's a very long thread about ghost inputs from 2017 (before the 3B+). There, @Brunnis found a hardware issue in the iBuffalo gamepads and fixed it by replacing its capacitors with stronger ones. It seems like their power supply would cause jitter in the BT signal which the Pi recognized as pad input, wheras the dongles known to fix this seem to filter them out.
Just FYI and in case you want to try a similar fix on your 8bitdo controllers (invalidating their warranty by doing so, of course).