Chain falling when shifting from small to big chainring
Your front derailleur is 'undershifting', meaning that when you release the pressure on the lever, the FD moves a tiny bit inwards. This throws the chain off the big ring towards the small ring. If the chain is on a smaller rear sprocket you might notice that it is rubbing slightly against the outer plate of the FD.

You need to adjust the HI-screw on the FD. Give it a turn anti-clockwise and check if the chain stays on the ring.

As @Criggie justly points out, when a bike's settings are adjusted on a workstand things change a bit when it is ridden. Mainly due to the slight deformation of the frame when under the load of the rider.