Issue with FieldSet in Gui Designer

I tried to use the Gui Designer page to create a new design
I need a fieldset on this one
But the fact is, each time I add the fieldset component in “design” view, switch to “code” view and back to “design”, the fieldset component and its content “disapears”.
Same thing if I just add the fieldset, go back to projects page to save the design, and re-open it. The fieldSet component is replaced by a basic template with “You can place any widget here…”

Using MacOsX 10.15.7 (19H2026)
Tried with Browser Firefox 117 and Chrome Version 116.0.5845.110