This girl happen the same thing, not sure if her solution worked, but for me it's been frustrating trying to change the text to bold, when it change it all for bold. It's frustrating that we paying for a software to have this crappy issues. I use Macbook Pro Big Sur. I have seen reviews from 2017 that have the same issue and doesn't seem to be a solution for the problem.
There must be some kind of bug. Could you please try to fix this ASAP, I would much appreciate it.
Sorry to hear about the trouble. To escalate this further, will it be possible for you to share a small video of the workflow? We'll also need the version of XD, we'll try our best to help you with this.