RoboHelp 2019: File names showing in TOC instead of topic names for most topics

When you said "latest RH 2019" did you mean New UI? I made that assumption but perhaps that was wrong. Which patch version do you have - are you both on the same?

I think if the topic doesn't have a title tag in the head section of the page, then the TOC will display the file name.

What I'm not sure about is how the topic would not have a title tag, as it is normally added automatically in Classic. 

Note that the behaviour in RH2019 Update 12 is a bit buggy, in my opinion, as using the Properties panel, it will look like there is a title. I don't think there are any fixes in Update 13 or 14 though, so I expect that you'll see the same behaviour.

For example, this topic looks like it has a Title according to the Properties panel (note I had to manually delete the title tag using code view to simulate the behaviour):

However, if I check the code, there's no <title>First Topic</title> code in the head section:

And here's how it appears in the TOC: