Those grey folder are in italics because they are read only my guess is your phone email program is creating them as they appear to be using the _ as a folder separator which would be very unusual

They have only appeared quite recently although the subfolders were created years ago and mostly not on the phone. Basically, on this particular account all subfolders appear now twice on Thunderbird: as normal ones and as those grey ones and I can't find a way to hide them.