I came across this forum bug while posting a bug for Pixelmator Pro.
If you have more than one occurrence of a word (let's use the word Pixelmator), and you highlight the second instance of that word then click on the Bold button, the first instance is tagged as bold, not the second.
Trying that now:
Highlighting the second instance of the word 'Pixelmator':
A place to talk about anything else with other Pixelmator users.