Arial Black
Adventurer
You've stripped this phrase of its context. It only applies to the beginning of the encounter.
It applies whenever the conditions are met; specifically: whenever you do not notice a threat when in combat rounds.
The DM determines surprise at the beginning of the encounter before a surprised creature has a chance to notice anything. Notice it doesn't say the DM determines when surprise begins or ends, just whether the encounter begins with someone being surprised.
The DM is obliged to determine surprise during combat, and that includes when combat starts. The first step is to determine surprise because that's one of the things the DM needs to know, but just because you must do this as combat starts does not mean that it only applies as combat starts. It (the DM determining surprise) applies all during combat, including as it starts.
You've been leaving out the bolded part. It's important. If a creature notices a threat after the encounter has begun it has no bearing on whether they are surprised or not as that has already been determined.
It has a bearing because 'noticing a threat' is the definition of surprise as a game mechanic. When you don't, you are surprised; when you do, you aren't.
Remind us, if you don't agree with the RAW of 'you are surprised if you don't notice a threat', when do you think that surprise starts/ends, and what rules support your view?