Lamplighter, I was a reporter for many years before going into PR. I've been the media contact for a lot of government agencies and private companies and organizations, Democrats, Republicans and other. Going off the record is never the preferred method of getting information from the reporter's perspective, and some reporters (and some PR pros) refuse to use it.
On the other hand, there are times when you either accept this arrangement or you don't get the story. Situations where this might be acceptable include: the source's job might be jeopardized, to avoid unwarranted embarrassment to a friend or ally of the source, that revelation of the source might put someone at risk of harm, that reporting the information is so important to the public that you'll do whatever is necessary to obtain it, etc.
I don't see how any of these situations applied here, but whatever. Let's assume for the sake of argument that the reporters agreed to go off the record on the mask requirement. Most reporters are wearing masks now anyway, so they likely didn't care about keeping this OTR.
Normally I'd agree with you that if a reporter agrees up front that information obtained in this manner should not be divulged. But as I see it, Pence's refusal to wear a mask, and his flimsy denial that he knew masks were required, constituted a breach of the agreement. The media's primary duty is to report the news, particularly when there's credible evidence of abuse of power, lies, hypocrisy, a danger to the public, corruption, or any other action that indicates compromised integrity. This situation, at the very least, showed hypocrisy and potential danger to the public. This fact is news, and this overrides any agreement to stay off the record.