I don't have solid steps to reproduce this, but I've noticed it enough times now to warrant filing something. Each morning, I open up my Nightly Firefox install that has all of my treeherder tabs open for sheriffing. I restore each of the tabs, and then use the "Set as bottom of range" item on something near the start of the day's pushes. My day goes on, with me classifying failures and backing things out. At some point, I start looking at what pushes would be good merge targets for a merge from integration branches to mozilla-central. These pushes need to be: 1. Free of bustage, including not having things backed out above it. 2. PGO builds/tests need to have run and completed either on this push or close enough that their results are likely valid for this push. 3. Jobs are all or almost all completed, with very few still in progress. So I look through the list of pushes, and find something that's either marked as Complete or only has a few strangling jobs in progress. This typically appears to be something from early in the morning. But if I refresh the page, a number of more recent pushes change from having hundreds of in progress jobs to only having a few, or possibly even showing as "complete".
Note, this only seems to affect the percentage complete. Actual jobs finish up and show as finished/failed just fine.
Have you seen this recently?
If it's still happening, I haven't noticed it.