I think it's a bug only, not intentional from Garmin to remove support to third party mapping and no problems with them on echomap.
English developer got the solution, but wasn't easy to discover it, that user has been lucky to find a knowledgeable support . Buggy is version 4.0 and bug remains after updating to 4.20, but as you could see, if you go directly form 3.40 (which works) to 4.20 then all is ok.
This affects all the units having in sw changelog: "Fixed potential combo corruption when updating software from 3.60, 3.70, or 3.80 to 4.00 and higher."
Probably the support you contacted stays in the dark about what's happening with these firmware This is "so Garmin" lol, probably you wont have further problems with next updates too.

PS. Have you made the hard reset after downgrading? This is important to clear NV completely after downgrading.
Thread marked as solved