Decent hypothesis, but not substantiated.
In 2020 the fraction of graduates who were STEM in China was 41%, Russia 37%, Germany 36%, Iran 33%, India 30%, and France 26% [1]. If we take the eleven countries in that article's GDP per capita, we find no statistically-significant relationship.
[1] https://cset.georgetown.edu/article/the-global-distribution-...
It would be interesting to contrast how much of them are STEM vs other "real world degrees" that get a job (accounting, hotel management or whatever) vs the "liberal arts" degrees.
> The WEF report identified China, India, the United States, Russia, Iran, Indonesia, and Japan as the top seven STEM graduate-producing countries in the world.
I think the US (and probably Germany too) is an outlier here because of the number of immigrants who arrive to study STEM degrees.
"About 30 percent of STEM degree holders living in the United States are immigrants" [1].
So sigificant as a fraction of immigrants. But not particularly meaningful to the trend. (I suspected the effect you hypothesise might exist within countries. But alas, higher-income households produce more STEM graduates [2].)
[1] https://spectrum.ieee.org/stem-immigration-diversity-gaps#:~...
[2] https://www.brookings.edu/articles/economic-inequalities-amo...