TechBoxNorth I can't say I've experienced this problem with debug APKs. But I have seen it previously when installing signed release builds locally. I assumed this was because I was installing release builds from a source other than Google Play.
If Play protect decides an app is something "suspicious" then it will show you a dialog during installation. If you pick the wrong option on the dialog your app ends up on a blacklist on the device for awhile, so installing will silently fail after that. The default option is to block the app.
There is also a global blacklist for applications which will block installation on any device, you have to appeal to Google to get this revoked.
If this to a change we have made it is not one we know of. There have been a number of modifications for that release, including a fairly major breaking change to the Mobile Advert plugin which we have talked about here.
It's difficult to tell if the 
joinner was having exactly the same issue, it sounds to not be related to Google Play Protect.
iggyplusceci your problem sounds completely unrelated to this. If you think there is a bug and have a replication then feel free to file an issue on our bug tracker. Mentioning a problem in passing on the forum is unlikely to get the attention of the construct team, and it's not enough for us to actually investigate a problem. I'm inclined to say that the Amazon QA team would not have rejected your application for being slow to start. There's a few reasons I can think of which could explain why the game did not load for them, but it's purely speculation.