To get the problem:
1. Click on a paragraph in the text.
2. Click the paragraph-format listbox (top left) and select e.g. "Heading 3".
3. Click on another paragraph.
4. Click the paragraph-format listbox again – it greys.
Clicking on the lower edge of the box sometimes allows the box contents to change.
The problem may be more intermittent than I stated above.
This is a VERY irritating bug. Please fix it as soon as possible!
I can reproduce this for BlueGriffon 3.0.1 on macOS 10.14.x, I don't see it on 10.13.6.
Hope this helps,
I don't propose to go back to an earlier version of macOS. Please just fix the bug.