Not really a bug but rather perhaps an oversight. There is a Bootstrap class (form-row) that should be added as a standard BSS component (Form Row).
See Form Row in the BS4 documentation for details.
If you add a FORM element and then select the FORM element, just above the Live View pane in the middle there is a FORM toobar which has a button for ROW which creates the div with the class form-row. It only appears or is an option when you have the FORM element selected.
Saj