@Miguel-Ruiz Fluthub appears to be experiencing some technical difficulties at the moment. It has been reported via Matrix and IRC.
Note that the stable version of Strawberry is several releases behind the 1.0.14 release.
$ flatpak search strawberry
Name Description Application ID Version Branch Remotes
Strawberry Music Player A music player and collection organizer org.strawberrymusicplayer.strawberry 1.0.0 stable flathub
Clementine is not URL encoding / decoding the file paths according to the specifications. Clementine is doing it wrong, and is probably unable to load the playlists saved by strawberry for this reason, since Strawberry URL encodes some characters.
Another difference is that Clementine is saving metadata. For Strawberry, there is a setting for enabling saving playlists with metadata, in the settings, under playlist. If you enable that setting it will save metadata like Clementine does.