So I'm updating this post because I don’t think my initial corrective action was the best. Here's the Original problem/conclusion:
While sailing in about 15 kts of wind (with the Square Top Main), the knot in the halyard pulled through the V-Cleat at the top of the mast. It happened again a few days later. I decided to mark the halyard at the cleat at the base of the mast at the raised position plus an inch to reduce reliance on the knot and v-cleat at the top of the mast. In this way the horn cleat near the base of the mast, not the V-Cleat near the head, was carrying the mainsail load.
I think it was not ideal to raise the sail an additional inch beyond the knot at the V-cleat. Why? The halyard doesn't stretch (by design). My action stiffened up the mast quite a bit because the taught halyard runs along the front of the mast. In this condition when you tighten the downhaul to flatten the sail, you don’t get the designed mast bend. I think the better solution is to make a bigger knot for the V-Cleat. After you make the knot bigger make sure you still have enough room from the knot to the head of the main to be able to pull the knot down to clear the V-cleat when de-rigging.
I think using the lower horn cleat to take the weight is problematic, the weight is intentionally designed to be taken at the top, you're changing the mast loading. The horn cleat is a backup, not a front line player. One thing I do is add whipping to the stopper knot to reduce chafing. Like some other people I have replaced the original troublesome wire strop at the end of the halyard with a dyneema stop, and have concerns about fraying in the v-cleat. So far it's been several years with no visible problems and the halyard strop no longer jumps off the sheave to get trapped between the mast opening and the sheave, which happened multiple times in the past on this 2010 boat.
I have had the same issue on my boat. I did tie a bigger knot and that seemed to help but I also lash the halyard to to horn cleat for added security.