Even leaving aside arcane protocol, only known to insiders, it was obvious to anyone that Meghan behaved inappropriately as follows:
Pushing, pulling and tugging Harry away from people he was there to meet and talk to. Lots of examples of this. She was bored, these people were nothing to her, and she didn't get that the whole point of her and Harry's presence was to honour them by talking and listening. Lots of instances of this, and its painful to watch.
Deciding the Royal Garden Party - where the whole point is for the royals to mingle and talk to the public - was boring after 15 minutes and getting Harry to tell KC and Camilla they were leaving. Even when KC protested and urged them to stay, Meghan insisted on going. (Can you even imagine??!! It's what she was there for, and the reason the taxpayers had funded her expensive outfit, the silly mare.She really, really didn't get it.) Also letting down her staff and the Fijiian ladies by bunking off early in that notorious market visit - so embarrassing for the UK. Royals just don't do that.
Always looking for, and directly at, the camera on royal duties. Royals don't do this (and neither do actors, in fact - they look at each other usually).
Actually giving voice to her resentment that she 'wasn't getting paid' for her royal duties. (No words!)
Walking in ahead of the PoW and P'ess of W during an official occasion with the late Queen, even when Charles specifically reminded them to stay back - and actually held out his hand to bar them! Meghan blithely ignored him and pushed on through, to the consternation of the Queen.
Deliberately 'missing' the coach for minor royals (after they'd stepped back) at the Platinum Jubillee, and instead arriving late so they could walk alone down Westiminster Abbey nave and get the special treatment they thought they deserved. Then huffing because they didn't like their seats and causing a fuss at the last minute.
So many more...but, you don't have to be a PhD in ancient royal protocol to know that the above misdemeanours are just due to lack of basic manners and consideration for others. She had no excuse, and Harry's a wet lettuce for going along with it.