had S9+ moved to S21 ultra EVERY mms comes with "no subject" these same people used to text me without that coming thru, it's not the carrier problem or everyone commenting would have the same one, either give us access settings like other androids have(Show Subject Field) or tell us why it's needed, same as others if you want us to buy in to your ecosystem fix this, it makes my horrible active 2 watch even worse with the best phone you have! it's not email it doesn't need a subject, work with you partners(carriers) and fix this.
Same thing for me. I went from a Note 20 Ultra to S21 Ultra, and every MMS or group chat in the stock messaging app comes in with no="" subject="">. My wife went from a S10e to the S21 and hers doesn't do this. We are in some of the same group chats and her phone doesn't show this and mine do.
Some of the older threads on this topic stating it is from the senders phone or network are since it doesn't happen on my wife's S21 and it does on my S21 Ultra in the same group chat.
My wife and I just switched from S9+'s to S21 Ultra's. We never had this happen on the S9's but now on the S21 ultra's we are both in the same group chat and she is not getting the <Subject:NoSubject> but I am. So it is not the other carrier, because we both would get that message.
This is exactly my issue on Our S10+. I've checked all my husbands settings for messaging and mine are exactly the same yet I get the <Subject:NoSubject> and my husband does not. When I first had the S10+ I did not have this issue. It suddenly appeared but has been going on for over a year now. Very irritating.