If the visa/RP is to be renewed/extended, its start date should be the day after the expiry
of the old one. I think that would ESPECIALLY be applicanle to EXTENSIONS!

I am referring to residential permits attached to FECs, not spousal residential permits.

What you think is not the case; the 30 day extension begins at the application date which must be at least 7 days prior to the end date of the RP.

It is not a 30 day L visa; the visa category is blank, only the final exit date is indicated: 30 days after the application date for extension.