TIMSS is done at younger ages (10 and 14) and the Finnish maths curriculum is obviously going to be behind because of the later start so the children are doing tests in maths they may not even have studied yet or only just started studying compared to countries that have been teaching those parts much earlier. It's nowhere near a measure of whether a country will produce 18 year olds capable of STEM because it's measuring too early - an international comparison of 18 year olds would be a much better way of seeing that as in Finland the more complex maths isn't learnt until high school, well after TIMSS.
Just found out something about PISA, it's a norm-referenced test so it's graded on a curve so other countries improving changes the curve which can lead to drops in points for countries that remain more stable rather than improving (although Finland has definitely dropped rather than remained stable as it was already realised in Finland by it's own research but the level it has dropped may not be what it appears)