Same happens with spring/torsion bars when you raise the trailer with the tongue jack.The chains absolutely change tension as the truck/trailer goes over bumps, otherwise raising the tongue jack to connect/disconnect wouldn’t have any effect on the chains.
The reason you don’t feel it as much is that it’s not an effective angle for the chains to apply enough leverage, this is also why it’s very ineffective at transferring a proper amount of tongue weight.
And the CAT scale on my setup says otherwise.