Post patch the velidreth set proc seems to often pass through the target without hitting at all. I've noted it doing this in nearly every situation including dungeons, trials, and target skeletons. Sometimes the issue appears to be related to the range at which you are from the target, but other times it doesn't seem to make any difference. I've included a link to a recording I made showing how the disease balls will more often than not pass through a target and do no damage at all.
https://streamable.com/r1xx1
I've been using the velidreth set since before the patch so I've seen how it's supposed to work and it clearly has something wrong with it now. On a final note, a guildmade of mine reported that his velidreth appears to be working perfectly normally, so I can't begin to explain why mine is doing what it is.