Don't know that you did anything wrong. From all I have been reading lately it seems to be a common problem with grafted plants whether you do them or buy them.

I know various types of supports are sold for supporting the graft point but it seems that it remains a weak point in the plant no matter how big/strong/old it gets. A real drawback to the use of grafted plants.

Some research seems to indicate that method of planting has some effect - graft point must be above the soil level and must remain above the soil level throughout the season. So mulching or any other activity around the base of the plant can create changing soil levels creates issues.

The method of splinting the graft point also seems to make a big different. Some clips apparently apply too much pressure and damage the epidermis, others don't expand enough as the stem diameter grows.

All in all, many possible causes, several beyond your control. Solution recommendations seem to vary widely.

Personally, it seems another example of the wasted effort and time except in those cases where one is plagued with the virus contaminated soil they were developed to resist.

Dave