Dispells can resist

This pretty much nails it on the head. This was a bug fix that didn’t make it into the patch notes.

In 1.12 and all of Original WoW, it was possible for dispels to be resisted. This was correct and intended in 2006, and simply went unnoticed until now in Classic. During 1.13.7 development we found the offending incorrect code and fixed the issue.

This means that offensive dispelling in pvp may not always succeed, particularly if the target you are trying to dispel is significantly higher level than you, as level disparity does affect this. Additionally, dispelling of harmful affects in certain pve encounters will also have a small chance to fail.

Thank you for the observations around this! But this is, in fact, intended and correct.