That is interesting, but the authors point out:

> In our study, in situ hybridization detected both mRNA derived from the vaccine and mRNA from the SARS-CoV-2 virus. ... our in situ hybridization method has high sensitivity and could detect trace amounts of mRNA, possibly reflecting unrecognized asymptomatic infections. These findings emphasize the need for caution in interpreting the presence of spike protein as exclusively vaccine-related.

We should also note that the study doesn't show that the original vaccine mRNA somehow survived for months, only that mRNA matching the vaccine sequence was detected by complementary probes.

I wonder if, in these cases, the vaccine was administered to someone with an active (but asymptomatic) COVID infection, and the vaccine mRNA was copied by the same RNA-dependent RNA polymerase that copies the viral RNA.

That might explain why both vaccine and viral RNA were found.