https://18xx.games/game/186098?action=54

I think it's because the private G5 isn't taken into account when deciding whether there are any valid actions.

"Each Stock Round, the owning player may purchase 1 CF&I share and/or 1 VGC share for one step cheaper than their current value. "

One step cheaper would have been $40, and I had $40 exactly. So I had a valid action and my turn should not have been skipped. Thanks for your help :)