You were only just diagnosed at the time, and had drastically changed your diet, and likely your blood glucose.
Changes in BG often go together with vision changes because our eyes adjust to the amount of glucose in our tears. This is only temporary though, glad to hear your eyes are fine now!
A hba1c of 6% or (42mmol/mol) is exactly on the threshold between normal and prediabetic in the UK, and just over the threshold for the US.
The flagging up is an automated system that doesn't take into account your diabetic status.
So while 6% for a known diabetic is pretty much perfect, for a non diabetic it's flagged because it shows something may be going on.