Likely cause
Climbing springtails that a seal alone will not stop
The answers point at one clear cause.
What to do
Many species gather on the walls and lids of culture containers, so springtails escape whenever you open a culture, even with gasket seals and mesh over the vents.
Paint a very thin layer along the wall just below the lid. A thick layer flakes off once dry.
Tip Give the lid a couple of sharp taps before you open the culture. That knocks the springtails to the bottom, away from the lid and rim, so fewer jump out.
Easy to confuse with
Vaseline, which springtails can cross or get stuck in.