Make sure the charger is connected to the laptop. Filevault encryption & decryption pauses when not using the charger. If you are using the charger, then perhaps you have a power related issue. Try running the Apple Diagnostics to see if any hardware issues are detected. Once the encryption/decryption process starts, there is no way to stop or cancel it.
Or maybe you have a file system issue. Try running Disk Utility First Aid on the hidden Container. Within Disk Utility you may need to click "View" and select "Show All Devices" so that the hidden Container appears on the left pane of Disk Utility. Even if First Aid says everything is "Ok" click "Show Details" and scroll back through the report to see if there are any unfixed errors listed. If there are any unfixed errors listed, you can try running First Aid from Recovery Mode, otherwise you will need to erase the whole physical drive before restoring from a backup or clone.
Filevault should not cause any problems. The only time Filevault may cause problems is during the encryption and decryption process because the file system is in limbo so some file system utilities and data recovery programs will not work. Otherwise, Filevault should not even be noticed or an issue. If you think Filevault is having an issue, then you most likely have some other issue (hardware or software). All of our organization's Macs (thousands of all ages) have Filevault enabled with absolutely no problems.
Try running the Apple Diagnostics to see if any hardware issues are detected.
To look for possible software issues, run EtreCheck and post the report here using the "Additional Text" icon which looks like a piece of paper. If you give EtreCheck "Full Disk Access", then more information will be included in the report which may provide more clues.
