Hello Igor
I tried using the Brush Script MT font (it's a fairly common font, I think you know it).
In TextEdit, I can't make the text bold.
I contacted a friend who works on PTE for windows. He did the same tests with the same sentence and the same font and he has no problem changing the font when he puts the text in bold.
On PTE Mac, I don't think this anomaly comes from the accented characters because it exists with both accented and non-accented words.
PS - I hope you understand me correctly because for me it is not easy to translate into English accurately.