I agree, this is a behaviour of Zen and Arc as well, and they did it intentionally, which I think is insane.
The whole purpose of pinned tabs is to have an easily accessible perma-bookmark. This means that users are likely to put super heavy websites in there - slack, youtube, twitch, notion, jira, teams. Which makes the need for suspension even more drastic.
When it was first announced in Arc, I had hoped that pins were basically PWAs that are pinned, which would mean they can be safely suspended, while still receiving notifications etc. I had hoped that the only additional mechanism will be a quicker startup/resume optimization.
In reality all the browsers copied cargo culted the worst version of this feature - normal tabs that you can’t suspend.
With that said, I don’t think Orion can change it, sounds like a Webkit thing to me?