Scientists not spawning to solve Strangerville Mystery

You could try creating your own scientist from any played sim:

In lifemode with the future scientist selected open the cheat console, type
testingcheats on
then
Traits.equip_trait trait_Hidden_Strangetown_Scientist

I also uploaded my household "Strangerville Extras" that contains a Strangerville scientist by name of Florian Kreitz. He kept that status when I moved him in. Tick "include custom content" to find the household (i don't remember what exactly is cc, I think some lab goggles on Florian, nothing essential).