Just ran into this problem too. I hope it will be fixed in the next patch!

This may or may not be related but I noticed that the D-Pad behaves differently when the joycons are attached. Specifically you can no longer read each of the 4 D-Pad buttons independently (4-key rollover). Eg. If you push 'up' and 'down' at the same time while attached only the 'up' will be detected as pressed. (controls(0).down == 0) Similarly 'left' will override 'right'. This does not happen when the joycons are detached.

That’s the way I would have done it if I’d remembered how. I like the way spt suggested though. The ’lock’ metaphor is easy to remember.