@terierul on Synology you can downgrade by uninstalling the app in Package Center and manually installing the version you want. It won’t remove any of the Plex data, I just tested it a few days ago and it worked perfectly for me.
You can find a list of archived download links for previous Plex versions here (just choose the file that matches your device/OS/CPU architecture).
Here are the full instructions from the Synology FAQs:
