There is something related to the file cabinet in 2019.2 where you can use internal URL meaning you can potentially access files without having the available without login checkbox... @michoel mentioned the other day but I can't find it...

Just to note for me our templates had image URLs like https://system.netsuite.com and changing to https://{account_id}.app.netsuite.com fixed - I only tried this as had an integration fail after generic system.netsuite.com endpoints were retired in place of account specific, and thought maybe something similar going on....