Stop start system, not stopping anymore.

For the last few months my stop start system has ceased to function. For a short while whilst it was still working I was getting a message saying I had to restart manually, but since then it has ceased to function altogether. A friend suggested that my issue might be related to my battery but it is not showing any other issues to suggest an issue. Please advise what I should be looking at to isolate my issue. Many thanks in advance.

I would start with replacing the battery. Most time if the battery voltage is below a certain level it'll prevent the auto start/stop from working.

Thanks for the suggestion but I would rather know what the "certain level" of voltage should be in order that I can test the battery first.

Typically any voltage below 10 is failing.

I have a 2015 Mondeo with Auto Start Stop and I didn't even know it was there. When I replaced a failing battery, one dead cell, so about 11 volts, it suddenly started working again. First time it happened at a busy intersection I thought I was toast. I had exactly the same issue with a 2017 Mazda 6, new battery and it started working again.
Some issues from ford.com to look for. "ford.com"

"Auto Start-Stop may not shut your engine off if your vehicle's battery is charging, the engine is still warming up, or if you are using the heat or air conditioning.
Your vehicle is equipped with a heavy-duty starter and battery to help compensate for a high number of starts and stops."

I feel even the incorrect spec battery may be enough to cause issues.