subpages are redirecting to the homepage

And there’s no redirection happening in the .htaccess file?

You may want to check the control panel at WPEngine and verify that there’s not a redirection setup in their panel. It won’t be a canonical URL issue as that doesn’t affect redirection, only indexing in search engines.

If I enter https://stgtrulite.wpengine.com/platform/?123 it redirects to https://stgtrulite.wpengine.com/?123 – so there’s still a redirect active somewhere.

As far as Google and their indexing, you can use their “remove from index” tool which would remove a URL from search results – https://support.google.com/webmasters/answer/9689846?hl=en

I’m not sure you actually want to remove it from the Index, just the redirection right? And if that’s the case, and you don’t have a plugin doing a redirect, then .htaccess or serverside redirection must be happening.