I have the newest firmware and I think the album art has become more reliable. I have the folder.jpg as well ...no embedded art. I like that the art is larger in the new firmware. I still have a problem with some albums saying no artist, no album. I have no idea what needs to be corrected on my end. I have been unable to figure out what might be different in the tags.
I find that a lot of MP3 files have tags embedded in both the ID3v1 format and ID3v2.3 together. A lot of DAPs either get confused or default to the v1 tags. The tagging software I use on my Mac (Kid3) shows the information from both tags if they are present and enables me to quickly delete the v1 tags and I’ve had no problem with my A50 since doing this.
Older software I have on an old PC laptop (MP3tag) routinely produces tags that confuse the A50.