Sometimes when I click Restore Defaults it creates multiple cloned entries of the Built-In Viewers, then when I restart a few times it perhaps gets pruned out & some Entries I explicitly made vanishes. I will report if it happens again, but I had 3-4 Entries in Launchers & 1 of them disappeared, & also some Textfile Editors I added. Needs to be more reliable, the File Type Setup, I'am just speculating, but if those multiple Entries don't appear when clicking Restore Defaults, then maybe the Entries won't get deleted upon next restart.
Even when I don't click Restore Defaults, it sometimes loses a few Entries.