I am seeing this on every run I have. Here are answers to your questions. I am using the new dashboard UI.
We use ray job submit.
I see this issue when there are more than 5-10 concurrent jobs where each job could be anywhere between 2K-12K tasks. The task count looks good for first few hours and then gets jumbled up.
Also the little hover popup showing task status on the Jobs UI does not show beyond 10000 tasks.