EDIT: SOLVED ! it was dumb user error on my side, check comments below

hello everybody,
i recently acquired two platinum compounders for peanuts, i really like what they do on many sources. I patched them on group inserts 1\2 and 3\4 of the desk (Allen Heath GS3 24 channels).
When i use one of them in stereo configuration, linking the compressors, i notice that the right channel is 180° out of phase with the left channel.
The mixer is all unbalanced at -10dbv so i used the proper connections on the compressors, for each of them i use two Y cables ( trs jack on the insert to dual ts jacks on the ins and outs of the compressor, with the switch in the -10 position).
Any ideas on what might be the cause of this?
thank you in advance.

My first thought would be to check the cables - it's possible the unit has developed an internal fault, but cables would be my first check.

By the way, I doubt very much your insert points operate at -10dBV - it's a common misconception that an unbalanced connection automatically means the operating level gets busted down to -10dBV.

EDIT: I just checked the user manual which states that your A&H GS3 inserts operate at 0dBu - don't touch that dial!

James, first of all, thank you for your advices but.... it was simpler than that, i feel so dumb... it was totally user error on my side... for some weird reasons the phase invert button was active on channel 2 of my motu ultralite mk3 hybrid, which is totally weird because i never touched them, if i have to mess with phase i usually do it inside the DAW...
i suspect it has something to do with the kernel extension no longer being officially supported, after a recent MACOS update i had to give permissions again to use an obsolete extension.
anyway, sorry for the loss of time and thank you again.