I have had this same problem with Affinity Photo.
Here's my work around:
Open a 'new' and select a document under the 'Print' heading.
Here, everything works the way it's suppose to.
If you create your Text here, you can Bold,, Italicize, whatever.
Copy the and paste into your project. You will now see that the Bold and Italics is not grayed out.
Here's an interesting fact:
When I created another text into my project, the bold and Italics were present. and, I could control the action.
When I changed to another Font, the Bold & Italics were now grayed out.
If I scrolled the Fonts, any that were italics, showed as being Italics. However, the B & I were still grayed out so I had no control over them.
I went back to the text I pasted into my project and the B & I were back and I had control.
When I went back to my newly created text, I re-selected the font I used when I created it, and the B & I were back and I had control.
Affinity Engineers: I hope you're reading this. This problem is fixable! please update for us..