By “Librem ad,” I’m guessing you mean the Librem 5 splash screen.
This “hang” issue is a known problem. Happens to me (and others) occasionally, too.
Apparently it’s because a process hasn’t completely stopped when we try to shut the phone down.
Note that “sleep” means something specific in Linux-speak, so you might want to clarify exactly what you mean, i.e. blank the screen by pressing the power button briefly, put in suspend mode, shut down, etc.
As with any computer, it’s not best practice, but sometimes you have to.