First the mailing list does not support binary attachments, so the screen shot is not available. (if you want to share those you need to push them up to an on-line location and share the url)
Ooops – it's here: <http://www.mynetcologne.de/~nc-dreszal/LO_Checkbox.png>, showing the toolbar, and the effect after adding the check box (“Markierfeld”): it appears, with the default title, but /without/ the actual check box control.
As for the form controls, they have changed homes - some of the controls that in earlier versions where on the 'More Controls' toolbar are now on the 'Form Controls' toolbar.
The effect also appears for a bigger project for accessing a PostgreSQL data base. The forms document has been created with an older Libreoffice version (5.2, iirc), but it worked just fine (apart from bug #115619) with 6.0, too. With 6.1, the check box controls are gone, with only the titles remaining. Apparently still operational if I click on it (the field in the database changes), but no visible control.
I'm using Ubuntu 18.04 and Libo 6.1 RC3 from the tdf download, I'm having no problem here working with check box controls on Base forms,
O.k., will try that version (at work, i.e. tomorrow).
So, what might be different from your environment and mine - I'm using GTK2 not 3, what about you?
I use XFCE on both the Debian and the Ubuntu boxes, with both Gtk+-2.0 and Gtk+-3.0 being available.
Thanks,
Albrecht.