I don't know what to think. I doubt we know the half of this. Apparently Mandelson didn't know he'd failed the vetting, and we don't know why he failed. I think it's perfectly possible that Starmer didn't know, but that begs the question of why he didn't make it his business to find out. Is there something still to come out, or that won't come out for 40 years or whatever? Possibly to do with Andrew.
If not for the Iran situation I think I'd be expecting him to resign, even if there are things he can't be open about, but I think he's doing a great job with the international situation, and when I think of what Badenoch and Farage would have done my blood runs cold.