I have the same issue, using (default?) monospace regular size 10, on pop_os 20.04 (replacing 19.10).
The 2nd line there has underscores instead of spaces.
Changing the font does work as a workaround, but monospace is a good default :( I added line_height=0;2; to ~/.conf/geany/geany.conf but it didn't change anything. @Xianic did you have to activate it somehow?