At least half my flats these days are ar the juncture of the valve and tube. There was a design change some years back, and the current design depends on a glue bond between the base of the valve stem and the tube. When the valve is wet, it corrodes and the bond fails. These are tricky and can be intermittant, sometimed holding or leaking based on the valve angle.

However, I get a fair number of these, especially the morning after riding in the rain.

My current fix is to fit a small O-ring onto the valve before installing the tube, and a second on the outside under the locknut. These seem to be keeping eater out of the problem area, and failures there are way down.

Now I's back to mostly punctures, but the summer was fairly dry here in NYC, so I don't know what winter holds.

Anyway, next time you get a flat, remove the tube, inflate it, and flex the valve around to see if there's any separation there.