The reason is that you are editing a MO file, not a PO file.
MO files by design don’t include untranslated strings, so when you save and reload you won’t get any of those new strings back, because they’re not in the file. Try translating one before saving and you’ll see that count go up from 1,848 to 1,849.
I guess the author has not shipped a PO file with their MO file. Not very useful, if so. But after the save operation Loco Translate will have created a new PO. So Sync and Save, but then go back to the dashboard and find it again to edit the PO instead. I think that will solve your problem.