The accesses in TILESTORED are as defined in the EAS : the rows will not go out of order and the dwords in the row will not go out of order. I think this makes the definition of overlap relatively obvious and same as a scatter : the oldest store will merge into the previous store. But if you have an example that helps me understand the concern?
Quoting from the SDM for scatters: "If two or more destination indices completely overlap, the “earlier” write(s) may be skipped." To me, you saying "will be merged" is not the same as "will be skipped". I understand that generally overlapping stores (from TILESTORED or any other insn) will be merged. Fully skipping (part of) a store is a different thing, though, and then also raises questions towards exception behavior.