Samsung Galaxy S6, Note5 users confirmed:
You need to keep grid size at 4×4 in the stock Touchwiz launcher
for compatibility mode to work.
The new Touchwiz launcher allows different grid sizes, but the widget size is not reported correctly if 4×5 or 5×5 is selected. Bubble Cloud Widgets relies on the size reported by the launcher. With my app the grid option is really no longer needed…
Thanks to Rosana for the tip, and Carolyn for the confirmation.