I'm thinking one of two things, or both.
Morphine definitely has a shorter half-life compared to PST or PPT. But there are slightly different elimination times for acute v. chronic use. I'm not exactly 100% sure, as I've not looked it up in a long time, but I think acute use stays in your system a tad longer than chronic use. Or, it could be vice versa. Maybe you'd need a few more days to test negative.
OR, it could be the PST. PST has a ridiculously long half-life. If you were a chronic user for a long time, and just quit 25 days ago, that could be the reason you're still testing for opiates.
On a side note (for PST users). Whenever I had a UA while I was drinking that stuff, and they did a more expansive test for other things like oxy, hydroC, etc., I'd always test pos for oxycodone in addition to opiates in general. I think it must be b/c of the thebaine in the tea, which is a precursor used to make oxy and other semi-synthetic opioids.