Can't See Printers from Certain Apps

[Douglas Summers]

(I apologize in advance; I remember asking a similar question a few
months ago, but I couldn't find it in the list archives)

I was trying to help a Linux Mint 18.3 user with a printing problem
with LibreOffice (can't see his local USB-connected printer) and I was
reminded of an issue I had (and still have) with Okular not seeing my
WiFi-connected printer. I tried Okular again today to see if the issue
was fixed, and sadly it is not.

Now, for me the printing issues with LO have never come up (been using
Linux Mint since version 18.1). I had the user try upgrading to the
latest version of Flatpak via the PPA, but it didn't make any
difference. He's also having the same problems with Scribus.

I know Flatpak can give apps additional permissions - can someone point
me in the right direction?