This happens to mine every year, but whatever it is doesn't burrow into the stems but the forming buds. Its so annoying. My flowers never open

I would say slugs or snails and especialy with all the rain we have had , I have had this happen to mine I snaped it off and found one inside the stem !!

Try a beer trap as using slug pellets with the weather we are having is useless, just though it may only be a small hole at firts but with the plant growing quickly with the damage done it does tend to get bigger and let more pests in

Gg

Something i never knew... touch wood mine have always been ok although mine are not grown in any of my borders which i find slugs/snails in quite regularly.