In 28.5, I can pretty quickly break the PAR2 tab by simply moving things around in the queue. At some point, it stops even checking (at least no new blocks show up as each file is downloaded, and all subsequent collections stay at 0 blocks). Occasionally I forget not to move anything and I want to downloading collection E before D, so I'll move those around while downloading collection A. It isn't 100%, but when I do that, that's mostly why the PAR2 tab stops working for me. One of the steps I try (I haven't verified it works all the time, as I generally just try not to move anything if I can because of the bug) is to stop all downloads (turn off the connection button and let it finish), move things around as desired, then reconnect.
When I'm moving things around in the queue while downloading, my system is probably backlogged because my line speed is faster than my computer can handle. Thus, over the course of downloading a DVD-sized post, it's not unusual for me to have (say) 10 "decoding" items in my queue for collection A while alt.binz has moved on to downloading collection B.. That may have something to do with why moving things around hoses the PAR2 tab on my system.