WvW participation bug?
Are you sure that you aren’t just moving up to a different level of participation tier? The progress bar is per-tier, so if you are not capped for participation then it would make sense to move from 90% to 20% after earning WxP (you filled in the last 10% of the current tier and earned 20% of the next tier with the sentry cap).
When you start out you are at 0%, and then work your way up to 100% for the first tier (25 track points I think?). You then start back at 0% and work your way up to 100% again to get to the next tier (60 track points). This keeps repeating for each tier until you get to 195 track points and cap out at 100%.
What you’re describing sounds exactly like this.
I think it is. Someone explained it to me and I came back here to say so. I’m a bit embarrassed now, lol. Can you tell I don’t WvW much? :p