Applying a bold span

I’m formatting some text from a Q&A feature where the first sentence needs to be bold. Now I can select the first couple of words and make them bold just fine. I can select, say words six through to twelve, all good.

However if I try and bold the whole first sentence, it will change the entire paragraph. It’s utterly maddening!

Has anyone got an idea of what’s happening here and how best to get around it? My best solution right now is to bold it and then manually reset the rest of the para to the correct weight but this is a faff and obv not how it’s meant to work!