So I don't have that bug "Colors/Brightness-constrast" it works exactly as intended, when I click Ok i got the contrast in the that I saw in the "Preview"
Having said that, and how about to increase the font boldness when the font text cannot anymore as it it at full bold, this is how I do it
Once my text is written, I right click on the text layer > Then Alpha to selection
Then Menu/Select/Grow... > input the number of pixels you want to grow
(it's always a good thing to save your selection (Select > Save to Channel)
Then New layer > fill the selection with the color you which (drag n drop a color from your FG/BG or use the bucket tool), you can also use pattern or even a gradient to fill the selection
Having said that, and how about to increase the font boldness when the font text cannot anymore as it it at full bold, this is how I do it
Once my text is written, I right click on the text layer > Then Alpha to selection
Then Menu/Select/Grow... > input the number of pixels you want to grow
(it's always a good thing to save your selection (Select > Save to Channel)
Then New layer > fill the selection with the color you which (drag n drop a color from your FG/BG or use the bucket tool), you can also use pattern or even a gradient to fill the selection