5.9% is 41 mmol/mol, which is technically "normal" BG in the UK. There is a standard acceptable error in HbA1c tests, which can be whatever the authority setting it thinks is OK. It's usually somewhere around 3-5% - so at 5% a BG value of 50mmol/mol could return an HbA1c of anywhere between 47.5 and 52.5 and be OK. In the UK if your result is that close to the automatic diagnosis figure they will invariably repeat it for confirmation.
Although all countries will automatically diagnose T2 diabetes with an HbA1c-tested BG level of 6.5%/48mmol/mol, there is no such agreement on what constitutes "normal" or "pre-diabetes" (which isn't a diagnosis). Pre-diabetes just means not normal BG levels, but not yet at automatic diagnosis levels. There's nothing to stop a T2 diagnosis being made sooner, but the practice in the UK now seems to be to only diagnose T2 by HbA1c of >48.
Some countries have also fairly recently decided arbitrarily to change their definitions of what "normal" blood glucose is. I've not seen any explanation for this, other than it being part of the general over-medicalisation of normality. The graph above shows the BGs of non-diabetic people and where the "normal" line gets crossed isn't obvious. I suspect they originally just went with normal/end of normal/start of T2 at 5.5%/6.0%/6.5% because the figures were neat. They don't translate exactly into mmol/mol either, so there's a bit of wriggle.
But it also depends on your doctor. Some people (I'm one of them) say that their doctor didn't mention that blood glucose was rising and was no longer in normal range. Other doctors (maybe more recently) seem to intervene earlier and recommend (eg) carb reduction, exercise, etc.
What you do is your decision. Many people have/had absolutely no symptoms when BG goes out of normal range and having to reduce it can seem a bit pointless. I got bad symptoms very early on (BG 43/44ish) but was firmly and wrongly told it wasn't diabetes. If your BG is rising, you might want to do something to stop it rising further. If it's actually stable around 39/40/41 (ie within test error margins) and you're just one of those people on the right side of the graph, you might come to a different decision.
Best of luck.