Can't sync my annotations on PDFs with dropbox

I just got a Samsung Galaxy Tab S7 primarily with the purpose of annotating PDFs from dropbox and saving them (annotated) back to dropbox. This worked out for a couple of times but now it's not working anymore.

I open the PDFs from Dropbox using Acrobat Reader and create some annotations. After I exit the PDF the annotations are saved in the Acrobat Reader "cloud", but they're never saved on Dropbox. That is, if I open the same file again from the Acrobat Reader cloud I can see the annotations, but if I open the file from Dropbox with any other app I can't see the annotations.

Any ideas?

Thanks