The name of the checkbox is confusing. “Transparent”, in the design field, means something totally different that what it does in Acrobat. Transparency can have degrees, and not so with a checkbox. The industry standard term that should be used is “No Fill”.

I wrote further explanation about why this is so confusing. http://documentgeek.blogspot.com/2019/09/acrobat-dc-how-to-have-drawing-markup.html

Users are clamoring for the ability to have No Fill, and they don’t know that it already exists because the naming is inconsistent and the feature is only implemented in 2 of the 4 different dialog boxes that are available to choose colors.

Attached are each of the four dialog boxes, each with a different way of representing “No Fill”.