Big meta studies may miss this because its a relatively new trend, and IQ is full of a million biases that can just generate noise when combined. When people are young, their IQ is significantly influenced by environmental factors, but as they age genetic factors become dominant. And in countries without some form of compulsory testing, sample biases abounds.
As for the past gains, look at height! Height is reasonable proxy for nutrition and globally it's just skyrocketed [2]. Japan is one of the more extreme examples, with their average height increasing by about half a foot since 1896!
[1] - https://en.wikipedia.org/wiki/Flynn_effect#Possible_end_of_p...
[2] - https://ourworldindata.org/grapher/average-height-of-men?tab...