The app version has a bug that drains Boost if you switch drivers while holding the Boost button. If it was the case, then click on the Boost button again on that driver and it will stop.
This helps if you want to do a fastest lap attempt with a weaker driver down the field but has a lot of inconveniences to it. Gl!
It's one car league I was on my driver when i pressed the boost it went to 95 then 2 laps later it drained on its own and i never pressed it I pressed it again but nothing happened it went down to 0!!