I've been using Inkscape to generate right-to-left texts.

The process has been:

• Typing an text
• Path/Object to Path
• Path/Combine
• Path/Reverse

For some reason it stopped working - after I perform the reversal, when I look at the "Edit" menu, the last operation that appears next to "Undo" is "reverse subpaths", and indeed the path reversal only changes the writing direction within individual letters, but the text is still rendered from left to right.

Any ideas what might have caused this weird problem?

Sorry I can't understand the problem:
you have a compound path from a text's characters.
And you want the characters following eachother from right to left, by reversing the path's direction?
Or to mirror it by that?
Reversing a path's direction alone won't make it have much difference in rendering.