It's a weird and ridiculous story, any way you look at it. Even putting to one side everything we have observed about Meghan and Harry--Meghan at least has never shown any signs of being the tantrumy type and we would surely have heard about it if she were--It's highly unlikely that Her Majesty had not given some thought to which tiaras she was willing to make available to loan. The Queen is interested in and knowledgable about all her jewels. My best guess would be that either she or someone like Angela Kelly showed Meghan and Harry a preselected group of suitable tiaras and Meghan made her choice from those.
And for those who think this is some kind of slander personally directed at Meghan, once again, Catherine faced the same kind of baseless, made up stories, and so did and do other royal ladies. It's the tabloids trying to get their readers whipped up into a frenzy.
I remember the stories about Kate's choice of the Cartier Halo tiara and how relatively modest that was compared to other tiaras the queen obviously has. As Catherine was to become the future queen, I believe she could have gotten another tiara easily, seeing those Eugenie and Meghan wore. Plus the tiaras HM loans out are usually very fitting to the personal style of each Royal lady, so I guess there is a procedere where HM selects some options (surely after having talked to the bride/wife about what they could imagine wearing) and then there is a personal choice.
We've seen Catherine now out in public so often and IMHO the understated elegance and modern design of the Cartier Halo really fits her. But now she wears larger tiaras due to her position which after three kids and an quite old queen is much more established than it was at her marriage.
Equally Eugenie: her parents wanted a "BIG ROYAL" wedding for her (did you read about Sarah calling Jack Eugenie's "consort" - poor man!) and her tiara matched that idea. Just like Meghan's matched her personal style very much with the more glittery tiara equal to the fun wedding they had.
So, no, I don't believe this story! Harry has so often said how much respect he has for his grandmother, surely Meghan learned from him to be careful with the old lady. Hm, will look up that Lainey article though. Maybe there is a POV that is worth looking at.