Underlay adhesive loosened

Hello everyone,
after Sabine also came by, I took a look under my roof structure today. I noticed that the vapor barrier is no longer glued between many rafters. If you pull the overlapping side down a bit, you can see yellowish residues there, presumably from an adhesive. At such a spot, you can also feel a slight draft.
The attic is not finished, but 20cm of Styrofoam is lying on the floor as thermal insulation for the ceiling below.
What is the correct procedure here? The easiest would be to glue the lower membrane to the overlapping membrane above on one side with suitable tape. But is that sensible? Better to ask a master carpenter?

Option a) Insert vapor barrier adhesive from the cartridge and bond it. Option b), easier: Siga Crall and tape.

As nordlys writes, glue the loose parts back together with Siga, Alfa Flex, etc.

You can also just leave it as is. The purpose is to keep blowing snow or horizontal rain outside. The railway should also ensure this with a loosely fitting overlap.

If you want to use Siga Crall or similar: there is a special one for underlay membranes, which is definitely better than the usual ones for the vapor barrier. We were able to test it recently at my father's place. I would probably just leave it as it is, though. It wasn’t glued for 20 years at my parents’ and there were zero problems.

Adhesive tapes for vapor barriers are also not approved for outdoor use, so get suitable sealant/adhesive tape for the underlay membrane. Even if it costs 10€ more in the end, I would—if the print on the underlay membrane is visible—stay within the manufacturer’s system. Then you will have peace of mind for a long time again.