Subform formatting using new UI is a mess

Subform formatting using new UI is a mess

I am trying to customise the size of the fields within a subform. For some reason, the titles adapt to the entered size, but the input field seems to default at 150px. I've tried this on Chrome and Edge and it does the same thing, see the image below for an example. This really limits the usability of the nice new UI.

Help!?