Welcome to Toolset support and I'd be happy to assist.
I've performed some tests on my website and I was able to reproduce this behavior. The field label is ignored when a checkbox type field is added using the visual mode and the field's slug is used as the label instead.
The field label is not ignored if the same field is added using the expert mode.
I've shared these findings with the concerned team for further review and will keep you updated through this ticket.
For now, a workaround would be to switch to export mode when a checkbox type generic field is involved.
( you'll see the option to switch to export mode at the top right corner of form's field editor - screenshot: hidden link )