IMO if they cannot give you the room category that was confirmed in your reservation, they should offer the opportunity to be walked to a nearby hotel that does have such a suite available. Of course, the first approach should obviously be to upgrade you to a higher category suite.

IIRC Sheraton Boston has some one bedroom suites that are pretty horrible, dark and on low floors with views of a brick wall or something similar. The suites themselves and their furnishings are at best just basic two room units that aren't particularly attractive, so their best nonsuite room could be preferable if you don't need the separate room for a business meeting, etc., although this isn't for the hotel to determine if you reserved a suite.

This is a convention hotel that I suspect can move furniture to turn connecting rooms into a (lockoff type) one bedroom suite, so I wonder why this wasn't done for the OP.

Twice in one month is making me wonder whether the OP booked a fairly good rate for the suite and then someone later was willing to pay rack rate for a suite, so the hotel sold the suite out from under a loyal customer. It could be that a previous guest overstayed, but twice in one month would be highly unusual. The hotel's reference to the entire stay might suggest that the problem is how specific rooms are allocated rather than not having some suite available for every night of the reservation. Perhaps moving once during the stay (with a significant apology gesture of course) would have been a solution.