As it was discussed in Solus 4.6 Release Thread, for some technical issues an up to date Solus identifies currently as 4.5, even through it is actually 4.6.
Having 4 DE's installed might be a recipe for trouble.
In your Initial post, you mention 2 login managers, one of them GDM.
Are you using GDM?
Cause this sounds a bit to me like an issue where GDM automatically sends your computer to suspend if you haven't logged in within 5 minutes.
Might this be your issue?
In this case pressing power button might wake you up from suspend.
If suspend is working on your machine.
Since suspend is prone to break, it's just a stupid GDM default.
