Disc of front brake rubs under force I ride a road bike with carbon frame and fork an hydraulic flat mount disc brakes. A fews days ago the front disc was slightly rubbing on the braking pad, so I readjusted the position of the braking saddle (loosen mounting scews, pull brake, thighten screws back accordingly to producer manual Nm). This worked fine and the rub is gone and hasn't come back.
However, when I put force on the handlebars - and thus the fork - like pulling handlebars when restart after red light stop or when ridding out of the saddle, there is a slight rub when applying force. It immediatly stops rubbing when ridding "normal".
So should I be concernd about this/reajust the brakes? Or is this just the fork flexing under pressure and nothing to worry about?
Flex causes this and it's not going to hurt anything. However, it's frequently the case flex is just the contributing factor that pushes the system into rubbing.

Look down at the gap between your rotor and pads, and slowly spin the wheel. If you were to see consistent .2-.4mm-ish gap on both sides, a true rotor (no warbles), and good parallelism between the pad and rotor, there's not really anything you could do to improve things at that point, and you'd know it was just flex causing the rub.

If you see an out of true rotor, off-center pads, or very minimal gap all around, then that means the rotor is getting closer to the pads in certain spots than it should be able to, and while it may not rub unloaded (in the stand etc), adding flex is getting it there. So to solve it you may need to true the rotor, face the frame/fork, reset the pistons, and/or adjust the caliper position.