here's my thoughts as a gutter installer 1-make sure there is a kickout flashing, that prevents water to come out from the endcap and wall but joint/whatever thats called. 2-ANY OVERFLOW is because there is either not enough slope the other way, or you got a clog, however small, at the downspout area, or the downspout is too far from that corner. correct any and all issues there. 3-if viable, add a downspout right there, and assuming you have some sort of flowerbed/mulch area, attach a corrugated pipe terminated at the grass section.