My team is performing as it should again (I rushed the last construction before the match). So, I confirm that there is a good possibility that certain on going constructions have to do something with the strange performance.

Another possibility is floodlights. Many of the matches which my team played strangely was at night. So, it could be the floodlights (which I did not upgrade at that time).

Another possibility is something fishy from Nordeus as it seems that the team performance gets 'blocked'.

But coincidences... no.