Hi,
I’ve just opened an issue in github (bug: Error creating route when using multiple subnets in one vpc · Issue #7915 · localstack/localstack · GitHub). Unfortunately I heard about this forum only after creating the issue. 
But as I just started working with terraform, localstack and aws theres a good chance that not localstack is the problem, but me. 
I don’t want to duplicate all the information here and would like to ask if someone could have a look at the issue above. If it’s more appropriate to discuss this issue here I can copy the contents from github to this place.