I have a plumbing vent stack that has a plastic/rubber ready-made
flashing/boot. It seems to be installed correctly (the flashing is
above the shingles on the downward slope and below the shingles on the
upward slope).

However, I get a leak only when snow melts. Rain is no problem.

I suspect snow/ice is damming at the vent and accumulating upward and
below the shingles. Is there a way to prevent this? Can I use a
shingle adhesive to seal the shingle to the flashing and prevent water
penetration below the shingle?