I hear you. Forget calendar days. Settlement timings are always...

I hear you. Forget calendar days. Settlement timings are always calculated solely on business days. That would mean those 875 shares possibly settled late by two days, not four. (Yeah, my calcs were out by one day.)
It would be cleaner to see a single tranche of 875 shares in your original trade fill as the most likely suspect, but I doubt it's got anything to do with doctoring or any kind of conspiracy. Moreover, I'd bet it's most likely centred around some kind of operational problem/snafu -- possibly with your broker -- which got resolved by the back-office, but still resulted in late settlement/recording for those shares.
I know the sums involved are not great and I don't wish to trivialise it. Have you tried reaching out to Automic to see if they can shed any light? They'll probably (rightly) refer you back to your broker. Probably not worth the frustration, but if you're keen that's where I'd start.