I had the same problem for a custom font which only had font-weight: bold.
I applied the “bold” weight to 700 and it didn’t work on frontend. When I changed it to 900 it worked then, kind of a weird behaviour. I would expect to output whatever I am using. For example if i would add bold to 100 i would expect it to output the bold-weight for the 100 style (I know that this doesn’t really make sense in a real world scenario but just to describe the behaviour).
I have to add that I’m not really deep into (web-)fonts!