Battery of my iPhone 13 mini (iOS 17.7.1) has stopped charging overnight

I'm afraid I don't know what you mean by "stepwise charging results." The cable is an MRA-certified (or whatever it's called) and has been used for years without issue, so that's unlikely to be it. The device is a 2021 iPhone 13 mini, so long out of warranty. I'm satisfied with the battery life, as long as it charges when I plug it. It's weird problem.

Thanks for clarifying. No sign of that. But for a quick update, I called Apple Support and after vetoing their suggestions to update the OS, they proved very helpful and make a few adjustments and it's now working again. They remotely connected to my phone and via screen sharing poked around a few settings. He disabled a VPN profile. It wasn't active, but I was no longer using it, so he got rid of that. Next thing, he switched the phone date and time setting to 'manual' (instead of automatic). We left 'Optimised Charging' off just to gauge the impact of these changes and hey presto, it's at least charging at night now. It's charged to 100% for two nights now. I'll try re-enabling Optimised Charging to see what happens.

Tune in for the next instalment of 'Battery Stopped Charging Overnight'. (Cue X-Files music.)