I've noticed the same problem in AC11.
The problem occurs when a Section has been converted to a drawing, then the 2d fills and lines are modified. When the drawing is rebuilt because of a change to the model, the modified 2d elements are deleted. Of course, any new lines and fills remain. The simple solution of course is to modify correctly so that converting to drawing is not required, some of us (ahem, me) aren't good enough yet to get the models to that point.
The solution? If you need to rebuild a section that is a drawing, copy all the elements first then paste them back in.
ArchiCAD 26; Windows 11; Intel i7-10700KF; 64GB RAM, GeForce GTX 3060