Play within disc brakes?
 

Basic Shimano BR M446 brakes. Centerlock rotor.

If I grab the brake while not moving, I can rock forward and back a bit.

Everything seems tight. I double checked. The frame isn’t cracked. The rotor body isn’t cracked. I cant feel play in any individual component.

Yet, here we are.

My other bikes and the front of this one don’t do that.

I think it’s just inherent flex, 29” wheels, post mount adapters, mid range components. Should I investigate further or is this ok?

My center lock - equipped bike did the same thing. Might just be a little pad play, but I wouldn’t worry about it unless it affects performance in some way.

The slop could be in the hub bearings.

That would be terribly obvious. It's either pad movement in the caliper, a loose caliper or a loose rotor. If it were hub bearings putting the brake on would make it impossible to feel.
ETA: The obvious loose headset or maybe fork bushing wear, or loose q/r or thru axle.

IME I could only feel it while not riding and locking the brake and "jiggling" the bike foreword and back - there was slop somewhere in the system and it developed over time. Maybe it wasn't the bearings but something else in the hub. New wheels fixed it for me.

edit: slop between the pads and the calipers makes sense, but in my case it was more than that.

It should not be very hard to do some forensics on the source of the movement. With the bike on the ground, or stand, engage the brake and try to create some movement in the system.
​​​​​​The potential locations are the rotor (loose bolts or lockring) the caliper (loose bolts) or some pad movement within the caliper (my bet).

Mine did this, I never figured it out and it didn't affect performance. I will say that it went away when I replaced the worn pads and the rotor.

Turn the fork sideways - lock the brake - and rock the bike back and forth. Place your fingers at the junction of the fork and frame. There should be little to no movement.

Pay attention to the hub and if MTB - the fork stanchions and crown.

Had that happen to me - the headset was not tensioned correctly - the other was a loose quick release.

Check the headset adjustment also.

Check spoke tension too