I found this (attached) on GitHub - DFRobot/DFRobotDFPlayerMini: Arduino library for DFPlayer
You'll see it is much clearer about the naming rules for files. (see 1.4)
It is always better to make an exact model of the SD card in a folder on a PC file system, then format the SD card, then copy the entire folder, without change, to the SD card. Do not then edit the SD card with the PC.
The mp3 player has a very primitive handling of the SD card file system and does not necessarily respect things like deleted file marks, file names (especially long file names) etc.