This is a really unintuitive load of settings. I spent hours setting understanding it and setting it up and have now saved a template with the HP mixes in.
What I ended up with is a bunch of busses called PH1, PH2, PH3, PH4 and assigned the headphone (HP) outputs to those busses.
Anyway, sounds like you got yours in a mess and need to revert to default.
There's also a "Default" button at the bottom left of each tab under I/O. You need to press it for each thing (Input, Output, Bus, Insert, Mic Preamps, H/W Insert Delay).
Alternatively - you could just try trashing preferences? May I recommend Pete Gates app for doing it.
https://duc.avid.com/showthread.php?t=340741
https://petegates.com/
Once you've fixed that, if you do want to output the same signal to multiple outputs, the Bus tab of I/O is the place to do it. You need to CTRL-click on multiple outputs in the "Mapping to Output" column. Each one you choose gets a tick by it. Absolutely not obvious at all.
Dominic