Little update.
I have released from slavery the good old CS2, the decoder reacts perfectly normal under DCC, i activated mfx protocol on it and it was perfectly read and worked properly. I putted again on the hands of the CS3 and it took several attemps to be fully read by the unit, once it was done, the decoder worked properly for a while and then again the same problem. Back to the CS2, the firmware version of the deco is from 10/2023, there is an update from 12/2023, even if it is no protocol bugs named in the changelog, i will make the firmware update tomorrow and see what happens....It seems to be a problem with CS3 software, strange thing is that it happens under DCC and mfx, so no problem with fit mfx compatibility. If problem persists will just have to wait to a CS3 update, i hope not to be the only person with the problem and marklin is aware of it. A quick search show nothing like that on the web though.
Regards

From what I understand, as others have mentioned, is that the solution is to set it up in DCC instead of mfx. I guess you will have to diable the mfx protocol in the locomotive's decoder in order not to have it automatically register in mfx when you are outside programming mode in CS3.
I haven't upgraded to version 2.5 yet.

Edit: after reading the posts from Francis again I'm not sure that what I wrote covers his problems. I may have misunderstood and the problem may be something completely else. Sorry if that's the case.

Is there any difference between the Zimo mx645, and mx650, and newer decoders being mfx compatible? Aren't they already DCC set up from Roco?