My 404 page works as expected except in the case of app.mydomain.com/user (the /user path was flagged by Google) in which case I’m getting a Not Found error. I don’t have a page named user.
Google also flagged me for instances where a subdomain I’m not using is entered (e.g., url123.mydomain.com). Not sure if this is happening because I’m using a subdomain (app) but I would like to clear it up if possible.

Hi @sydney22,
I am experiencing the same issue (also pointed out by Google Search Console) with the /user path.
Redirect to my 404 page work just fine for any other non existing path (page) but as soon as /user is added to the URL (no matter what follows) the 404 redirect doesn’t work and I just het a plain {“message”:“not found”} on the page. Did you figure out how to solve this issue ?
Best,
Thomas
No but eventually Google quit pinging me on it.
I reached out to the support and here is the answer they provided >>
To provide some context, the 404 page is not shown when the URL contains /user. This is due to the /user/hi being the endpoint called to keep the current user’s session active.
So I guess we’ll just have to live with that.