@Synthfox thanks for the report!
This should not be the case - could you check if this is a USB-only issue, or if it also happens, if you attach a synth to a classic MIDI OUT? If a synth on classic MIDI OUT works, could you attach a softsynth/VST via a computer to the USB port of your LoopA and test without the USB MIDI router inbetween?
I am using my MatriX with my LoopA on my Mac m1 driving VSTs in Ableton via USB - see photo below, taken a minute ago - USB forwarding works in this setup - are you running the recent v2.09b4 firmware on LoopA and v1.00b4 on MatriX?
Maybe the USB router filters some of the notes?
Best regards,
Peter