Because the UCJV330 uses circulation dampers for white, they seem to bleed a lot more than subtanks when idle, especially on circulation cycles. Not sure there's a way around it. Can check your capping station gap. Note that I did my first install on a UCJV330 just this past month, so I don't have a ton of experience on them.
Just looking at the capping station, it also appears much more prevalent on the screen because it's a thicker ink in general. Empty your waste tank and check on it in a couple of days. If it looks bright white, it might be an issue. If the ink is more of a milky freakish purple, that's pretty normal.