General update here: Until recently the suggestion from Redirect to Function - Splat is not working as expected - #2 by jonsully worked fine for me and the variable was filled as described. Recently that started to not be the case any more on some function runs, instead the string is just empty. No idea what changed on Netlify side 
Fortunately I could get rid of the param in my limited use case that was still live. Still confusing.