Problewm solved, I'll post the solution for future reference.
I usually work on my PC and then sync my PC with my Android devices, and frequently update some PDF just by replacing them with updated files (of course with identical names) on the PC library folder. It turned out that this was causing the issue. Here's Mike's answer:
"Yes [that can cause the problem], because MobileSheets won't have calculated the new file hash for those files if you change them outside MobileSheets. You have to do one of two things:
1) After replacing a file, load the song using that file in MobileSheets so the file hash is updated. You have to do this for every song using a file that was replaced.
2) Long press the song, tap the three dots at the top right and select "Swap File". Select the new PDF for that song. Use this approach instead of just copying songs into the storage location."
Using "Swap files" command instead of replacing the files direclty on the library folder works flawesly.
I usually work on my PC and then sync my PC with my Android devices, and frequently update some PDF just by replacing them with updated files (of course with identical names) on the PC library folder. It turned out that this was causing the issue. Here's Mike's answer:
"Yes [that can cause the problem], because MobileSheets won't have calculated the new file hash for those files if you change them outside MobileSheets. You have to do one of two things:
1) After replacing a file, load the song using that file in MobileSheets so the file hash is updated. You have to do this for every song using a file that was replaced.
2) Long press the song, tap the three dots at the top right and select "Swap File". Select the new PDF for that song. Use this approach instead of just copying songs into the storage location."
Using "Swap files" command instead of replacing the files direclty on the library folder works flawesly.