Migrate from US to EU data center

I am trying to migrate our instance from US to EU data center. The plan I have is this:

1. Set up duplicate tracking to run both instances in parallell (through GTM).

2. Once we have made sure data is being collected as it should in the EU instance, remove instrumentation for US instance.
3. Optional: migrate data from US to EU instance.

I have set up step 1, but the EU instance is only tracking a fraction of the evens being tracked in the US instance. Is there a known issue running multiple Amplitude instances on the same page (using GTM)?