About this issue, it looks like your theme is missing some specific theme parts/slices like buttonset_item_normal, buttonset_item_hot, buttonset_item_hot_focused, buttonset_item_focused, buttonset_item_pushed.
I think a possible fix would be this: 1) if the icon fit the status bar height, it’s drawn in its original size vertically centered, and 2) if the icon is bigger like in this case, the icon is resized to fit the status bar size (the down side is that some quality of the icon could be lost), 3) the theme could have a property about the status bar size (actually it’s automatically calculated depending on thte font height + UI Scale*8 pixels)
May I suggest maybe a 4)Have a different set of icons for the status bar?
I prefer 2 in this case, the result should be “good enough” since this is such a fringe case. ^^