I think I had this problem a couple of years ago: moved to a new phone, clean SD card, MyTrails app automatically copied across by Android. All my maps listed. Copied the …\offline folder to new SD card on new phone. Any attempt to access new maps failed (the SD card number after /storage/ is different).
Set the ‘where to save offline maps’ correctly … shows new SD card number; still no success.
Tried the ‘reset offline’ hidden option … still does not work.