UPDATE: It does work both Merlin and stock, for a time it connected in both circumstances, and its connnected now. But why?
The only thing that comes to mind, as I've been changing things. Is that I had one of the other SSIDs (Guest) with 6Ghz enabled at one point, and I just removed it and rebooted the router and it connects even with WiFi7 and MLO enabled on the main SSID
Now keeping VERY verbose and detailed notes going forward on any changes and rebooting the router between them

to see if it falls off again and stays off and what the trigger might be

Also going to try and join that client it to the Main SSID with WiFi7/MLO just to rule that out or in as the case may be.
Also need to check authentication method or WPA/WPA2 vs WPA2 vs WPA2/WPA3 (WPA/WP2 for the 2.4 and 5Ghz SSID andWPA2/WPA3 for the main MLO based one is what's working now)