First of all, I’m happy to report Juce8 does render ligatures for fonts that support that much better than I’ve seen before. I guess this is a positive side-effect of using HarfBuzz.
However it seems the Juce TextEditor component is getting confused by the ligatures and character counts. If a ligature is entered between characters, the caret jumps one letter too much and if you want to delete the second letter of a ligature the behaviour is inconsistent - sometimes the first letter is deleted instead of the second.
Here’s a video of me playing around in the DemoRunner Font demo with the Corbel font (included in Windows 11). This is on Juce 8.0.3.