Usually these things will MOVE underneath the leaves. Find the leaves that were cut out & that's where they'll be.
I'm not sure what your tank looks like but WOOD is the last spot they'll loiter around since green on a brown surface doesn't match & their hiding spot sticks out like a sore thumb.
Leaf miners and the the other worm like insects that turn themselves into insect cannolis in addition to the dragon fly larvae are some the hitchhikers I dislike the most. Hydra's can be a menace too but only if you have a fry tank.
Once you get rid of them (Check over a week or two for additional signs) you'll be THRILLED & then 1 month later you forget all about them until they reappear in the tank.
Find the camouflaged adults for your bettas, & hopefully they will locate the smaller ones before reaching adulthood & mine the rest of your plants.
- If you can get a decent photo of the plant that's missing pieces I can tell if you really are dealing with the "miners".
Remove the plant from the tank, put it by a window or bright light & photograph the leaves.
I've left my java ferns out of the water for about an hour or more with no ill effects to the plant.