Anyone else seeing fresh accounts in high-rated matches?

Account level by itself is a pretty weak signal. A new-looking account could be an alternate account, and a low level doesn’t tell you much about how the person plays. I’d look for repeated patterns across matches before calling it a smurf problem.

If you’re seeing several in one match and it keeps happening, jot down the dates and check whether the same pattern shows up in your next batch of games. That won’t explain the cause, but it helps separate a brief run of odd matches from something that’s sticking around.

Changing who can spectate high-rated games doesn’t sound like a clear fix for low-level accounts appearing in them. Those could be separate issues. If an account seems to be breaking the rules, use the game’s reporting tools rather than treating its level as proof.