We've known that forever. There's been studies done with luciferase to see where the mRNA triggers protein production in small animals.
We know that a big portion of the activity is in the liver, distant from the IM injection site (a lot of the activity is at the site, too).
e.g. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4624045/
"When mRNA-LNPs were injected intramuscularly and intratracheally, similar to intravenous and intraperitoneal deliveries, a large portion of the luciferase activity was detectable in the liver, demonstrating systemic spread of the nanoparticles."