Most slides have two seals. One is the actual seal (to seal out air, water, hopefully bugs and critters, etc. It is usually a rounded "D" shape.
The other seal is called the wiper. It is blade shaped and it's function is to wipe or squeegee off any water on the slide roof and sides
This is probably the one you are referring to that "folds."
This is normal and how they work.
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