Ruger AR 5.56

Just whacked my stock while the bolt was locked back for science and the bolt closed.

(Not a Ruger AR556, so it looks to be a platform normality)

The bolt catch is elevated by the mag follower and magazine spring. In the absence of a magazine, there is no activation force on the bolt catch, so if you bump the rifle enough to 'bounce' the bolt pressure off of the bolt catch, the bolt catch return spring will release the catch and the bolt will close. Since there is no magazine, unless you manually and intentionally placed a round directly in the chamber, the rifle will not be loaded, so the distinction of bolt open and closed in this method is trivial. Remember - the bolt catch has a spring such it wants to NOT catch the bolt. Only when the mag spring overcomes the bolt catch return spring will it activate and catch the bolt. That is why it "functions normally" when you apply manual pressure, but lets the bolt close when you don't, without a mag.

This is NOT a feature unique to the Ruger AR556, but rather a normal function of all AR15's.

It's normal, it's safe, it is NOT a design flaw, nor a malfunction. Don't lose sleep over it.

My son has ARs from different manufacturers.
If the bolt goes back (bumping the butt on the ground, retracting the operating handle), the last-shot-bolt-open catch springs out of the way.
If the Ruger AR556 did anything different than that, that would be usual.