Many MANY times a fix in ANY program is much more complicated than we outsiders can imagine. With this expanding into BNet territory, my guess is Diablo is NOT the only one effected.
However, fixing a bug is much more complicated, even at it’s easiest, than flipping a switch. It’s going into the code, FINDING the issue, then coming up with a fix. So, they fix a bug in area A, and all of a sudden area T starts having issues. Roll back fix of area A, T starts working. Initiate dual-fixes for A, and the subsequent issue in T, and all of a sudden area Q, J, and Z now have issues. Roll back the previous fixes. New issues persist, you establish previous fixes. EVERYTHING WORKS NOW and no one knows why.
From the classes I took on computer programming, this is the process.