Interesting never noticed that before.
The pins on the images in the film strip are toggled on and off by the menu item Show Place badges, under the gear,
Why that is toggled on when you select an image, view it and drop back to film strip mode is unknown. Doesn't seem to be a use for it. If the badges are a problem you can toggle them off by going to this menu. If you need them on before going to view mode you can toggle them on here.
You could report this to Apple Aperture->Provide Aperture Feedback as a bug.