Created attachment 157050 [details]
lml used for applying font attribute changes

SUMMARY
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Changing the font attributes of the plot title and axis titles work not as expected. 
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STEPS TO REPRODUCE
1. Load the attached lml.
2. In the plot title select the word Convective and change it to bold - works. Revert.
3. Change the C in Convective to bold - does not work. Revert.
4. Try the same with the H in Heat - works. Revert
4. Select any other word and change it to bold: Might work (9/10) or not (1/10)?
5. In the x axis title select "v" and try to change it to bold - does not work. Revert.
6. Select "[m/s]" and change to bold - works. Do not revert.
7. Select the whole string and toggle bold - won't work (_nothing_ is toggled in the Plot Area, you have to "untoggle" it piecewise, it's correctly shown in the property sheet but not in the graphical area).
8. I'd like to encourage you to perform your own experiments in the case I left something out.

OBSERVED RESULT
In summary it seems changing font attributes never works on the first character _alone_, no matter if it is followed by a blank or another character. Toggling attributes depend on the previous status."Never works" means changes are not visible in the Plot Area although shown correctly in the property sheet.

EXPECTED RESULT
See in the Plot Area what is shown in the property sheet.

SOFTWARE/OS VERSIONS
Windows: 10 20H2
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION