The UPS tracking API has a similar issue, when it doesn’t find a tracking number it returns 200 and an API response with the not found error inside. The documentation even states that the API does return 404 in that case. A github issue about that mismatch is open for over a year already: https://github.com/UPS-API/api-documentation/issues/166

I always used offline reader because the feeds i subscribed to don’t have that many updates (most of them are currently webcomics updating 1-3 times a week) so i generally just check on it a few times a day and none of it is really urgent, so there is not really much point in expanding the time where i could check on it. Another factor might also be that i started using RSS with Firefoxes inbuilt reader (which they removed back in 2018) which didn’t sync across devices, so i was just used to it being like that.