Inheriting systems can be challenging! This forum is a great place to search and learn 
First, please see 
Courtesy Reminder that FreePBX v15 is EOL Starting 1 October 2025.
As for the image, was looking more for something like Disk Space Display Issue in FreePBX After Mounting a Drive for Call Recordings (although the issue mentioned there is probably not what you are experiencing – it is a nice picture.)
You might also consider looking in to the logs for any error messages, as well as temporary files in /tmp or /var/tmp that are getting cleared out after a reboot. Do you have shell access ?
Generally speaking, NVMe is still a little quirkier than SSDs, especially on older Linux systems like yours. You are probably in the minority of users of FreePBX with that kind of setup – which points to potential other custom scripts and such that might be filling up temporary file holding areas (which are cleared at reboot.) Or, it could be some interaction with failed drive TRIM operations during normal operation, which only successfully complete during the next system startup – seems like a reach tho to get to that point right now, as there’s much other troubleshooting to consider.