In sans-serif text typefaces capital ‘I’ is traditionally a simple vertical rectangle without serifs. But we were estimating some folks would miss those serifs so if you want to have a ‘properly-formed’ capital ‘I’ just activate stylistic set 02 (ss02, = ‘Alt I J i j r’). To see what this looks like, take a look here: https://www.monolisa.dev/tester/text

Thanks for the reply, and that option. Pretty cool!

But the crossbars aren't serifs!

You are correct. But the letter ‘I’ never has crossbars, no matter what font you are looking at. If you mean the horizontal stem, that is a serif.