One of the big differences between natural immunity and first gen SARS2 vaccines is that the nasal lining (epithelium) is only protected with natural immunity. They have so far been unable to replicate this with the new vaccines.

Note that the article points out that with the delta variant the viral load within the nose is ballpark 1000x higher than for vanilla variants. Entry via nose is one of the vulnerabilities for the vaccinated; perhaps hence the mask guidance again, to prevent large clusters of the coronavirus depositing in that weak spot, as well as prevent symptomatic vaccinated from shedding via nose.