The Preferred Printing Tool is not working for every card or variant of a singular card. I spot checked roughly 10 preferred printing cards and found that 3 did not properly work. Either there was not option to change the preferred printing such as Battle Cry Goblin that has two variants in my collection or the option allowed me to switch to some variants but not all, such as Lightning Strike that has 4 variants in my collection and is seen in the attached .png
Additional point but not a reportable bug. Why can't we have a preferred card style as well? I hate changing the card style to the one I like and not the one that defaults in my collection when I hit the 'Apply Styles' button.
It seems that it is not working for any of the "fancy" styles for the cards, including extended art and showcase versions, or the Japanese style Mystical Archive.
Which defeats the whole purpose of this feature for most of the cards.
To use it for non-style printing you can click on the version without the heart icon and then select the non-style version of that printing and "apply". Then and only then you can favorite that printing.