@Damster
When no listen buttons are selected, the main LR mix is routed to the listen output, which by default is routed to the headphone output/s.
This is by design.
On larger consoles (e.g. Qu or SQ), there is an option to turn this behaviour off, so you are welcome to add a topic to the CQ feature suggestions section here on the community.
If you’re wanting to use the headphone outputs for a separate mix however, note that you can assign any of the individual outputs to the headphone sockets in the CONFIG / OUTPUTS screen.
Thanks,
Keith.