Stop automatically choosing / replacing featured images

Hi. I'm not sure if this is a GP issue or something with WP in general, but I'm having a problem with featured images.

I'm trying to set a featured image that's different than the actual in-post images (where I'll only use the featured images on archive pages).

But every time I manually set a featured image and save the post, the image changes to use the first in-post image as the featured image instead.

Any idea if there's an easy way around this? Maybe a setting I'm not seeing? Or is this likely a bigger compatibility issue with a plugin or something?

I don't have featured images set to display anywhere in my GP settings yet because I need to add more of them with our new branded designs. But here's an example post where I'm seeing this:

https://allfreelancewriting.com/coronavirus-pandemic/

The tall image is the first in-post image. But I don't want that set as my featured image for archive pages. I'm setting our Twitter post graphic manually as the featured image instead. This one: https://allfreelancewriting.com/wp-content/uploads/2020/04/Freelance-Writing-Coronavirus-Twitter.png

But when I set the horizontal image as the featured image and save the post, it automatically changes my featured image to the vertical one inside the post itself.

Here's a short screencast of the behavior I'm seeing on the backend.

https://www.youtube.com/watch?v=T6euWTvdrbc&feature=youtu.be