I use a floating group for my app menu. I upgraded a page to the new responsive engine and a floating group seems to move around.
Here is a loom video showing what is happening. The first view is the production app which is on the old responsive engine, the second part of the video shows the issue on the new responsive engine. Loom | Free Screen & Video Recording Software
It can happen when you have multiple elements on the screen, some of which are on the new responsive engine, some on the old engine. Migrate all elements you use on that page to the new responsive engine and the issue should go away.