I forgot the name but I believe it's a bug that bores into the stem.
In the picture. There's two very small dots above the damage. The dot farthest from the damage appears to have a light brown tinge around it. Have you zoomed in on those spots also? Might be nothing. Might be bug poop or bugs themselves.
No worries. It's a not an uncommon problem. A few people on here have had the same. Somebody will more experience on the stem damage will chime in. (Edit: Jim did)