Script for checking if arc size was changed, and changing it back.
Set arc_check.sh file to run with cron, InitScript or both.
In arc_check.sh change <CHANGE_ME> value to match the size you want. For converting GiB to bytes. I've used this site.
Go to System Settings -> Advanced -> Cron Jobs
Click Add
In Command filed you need write path that contains your script, as follows:
/dir/to/file/arc_check.sh
You can run this script as root, but you should set your permission up so that a special user can run and edit zfs_arc_max file.
In Schedule you set it to run every at minute 0 past every 4th hour.
00 */4 * * *
You can also uncheck Hide Standard Output/Error.
You can configure it in TrueNAS GUI under System Settings > Advanced > Init/Shutdown Scripts