Although axillary lymphadenopathy is a known side effect of COVID-19 mRNA vaccines, such changes are transient and contribute only to the perception of breast growth. To date, there has been only 1 published case outlining lasting parenchymal breast changes, presenting as a palpable mass, following administration of the Pfizer COVID-19 mRNA booster. No formal studies have established a link between breast parenchymal growth and the Pfizer COVID-19 vaccine.

It's a one off case and they were unable to draw any conclusions, like if it was the spike protein to blame or if it was the carrier solution.

A bit off topic, but, I had a Moderna booster shot and the flu shot both in the left arm a couple minutes apart - 20 .. 30 mins later I had a notable soft mass form (not from pus like a boil) on the underside of my left wrist (where one would take a pulse) about 7/8 in / 22 mm where it persisted for a year in that state, another year or so on slowly shrinking, (ie more than 2 years) is now approx quarter the volume. That would most likely been from some weird interaction of the vaccine carriers and not likely from covid's spike protein.

Correction edit: My recall is bad, I believer it was not a Pfizer shot (my first two were) but Moderna's vaccine.