It's almost that.

Adsense policy does not allow 300x600 ads above the fold on mobile where advertisers like to be.

On occasion auto-ads places one just under the header, above the content, and a sitewide temporary ban notice gets sent out. Google it for examples. When it happens core vitals stats get wrecked too with a big content paint and layout shift. Heck, the ad often finishes loading when a visitor has already scrolled past it. Not good!

Advertisers would love to bother but they know they'll pay top dollar for bottom of the page mobile treatment with the 300x600. Personally I think they are perfect for a desktop sidebar because the sidebar moves to the bottom of the page on mobile on most wordpress sites... but advertisers are better off fighting for above the fold spots,with smaller creatives, they cost the same on mobile.

With the apparition of anchor ads, at Adsense, there was a revival of the old horizontal banners :)

All of Google's auto-ads within the main content are horizontal on desktop aren't they? The sidebar and mobile is the only place I've seen vertical ads, granted I'm not looking for them when I browse the interwebs.

Go figure the 300x600 on mobile is the one causing problems.