Yawgmoth, Thran Physician's first ability covers up the rightmost creature during targeting and makes it impossible to click
Activating Yawgmoth, Thran Physician's sacrifice activated ability does not let the player select the rightmost creature from among the player's own creatures if a sufficient number of creatures are on the battlefield. The representation of the ability on the stack covers up the rightmost creature (or several, in some cases), and it's impossible to click it.
This unfortunately makes executing the combo with the player's own undying creatures impossible in some cases where the undying creature, such as Young Wolf, ends up being the rightmost creature after it comes back from the graveyard.
I reproduced this on the macOS and iPad versions of the client reliably. I noticed that the ability on the stack "minimizes" when the opponent has a lot of creatures, but if the player activating Yawgmoth does, the ability does not minimize and covers up the rightmost creature or two.
Yawgmoth's first ability should always be able to target any valid creature target, including the owner's other creatures, even if they happen to be shown in the rightmost position on the screen.