Tony

Is your mast by any chance raked forward?

I just realised that would make the sidestays more likely to catch the diamond tangs.

There was a time when Mosquitos (particularly sloops) had their rigs raked forward. These days they are almost never set forward of vertical, and any boat with the centreboard cases at their maximum aft position would have the mast raked back. Some cat rigged Mozzies (like mine) have quite a lot of aft rake.

Try dropping the mast back and see if it still catches.