In addition to the behaviour I've documented about the selectable feats: https://www.dndbeyond.com/forums/d-d-beyond-general/bugs-support/13975-inconsistent-feat-selection-box, I've found a couple other issues with the Modifier - add a feat option:

1. When added to an item, it does not appear to do anything. I see no option to choose a feat (if I select "choose a feat") nor do I see a fixed feat (if I select grappler) when I add and equip the item on a character
2. When added as a racial trait, it works. But if I add it TWICE, I still only get the option to select 1 feat. I can create a second racial trait and add the modifier to that as well and then I can select one feat per trait, so there's a workaround, but it seems silly.

Did I miss something or is this a bug?