A friend of mine boosted 7 necro.

He got another mareth reward

He went back and boosted 7 necro

He got the build 7 necro reward

He checked again, and now it said he only built 6/7

This is the screenshot:

He clicked to boost 1 minute

Message tells that 1 minute is more than needed.

So it seem boost is now -1m 53s in time

Even if he restart the game it still say the same