This MMS issue popped up months ago when S8 users first started loading Oreo on their S8s. For some the problem only seemed to impact them when on wifi, but not everyone. Are you trying MMS on wifi or LTE? If wifi, disable it and try on LTE. Are you using the Samsung messages app or Android Messages from Google? Double check that the app is fully up to date.
I haven't searched today, but I thought Google sent a fix back in September for the issues involving Oreo breaking 1st and 3rd messaging apps on some devices, not all. Oreo actually broke a few apps, stopped notifications, developers updated to fix incompatibility. It was so long ago tho. Do search it up.
Nothing to do with an update on my S7, but messaging for me using default, and for other people I know on Telus, has been very slow periodically the last couple of weeks--texts can sometimes take long minutes to send, and MMS can spiral for days before downloading, sending MMS is always fine tho.