AGM's do just fine with deep discharge, if they are designed for deep discharge usage, just like flooded cells.

Just like any battery it has to be used properly and be designed for the expected use.

That is why car starting batteries don't do as well as deep cycle batteries in deep discharge applications.

Not designed for it.

After all the industrial and commercial sized UPS' I work on use AGM batteries in deep discharge applications.

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.