Yes you are righ6t it's not 100% accurate, but I'm yet to work out what makes a font work when only the t10 version is there.
My experience has been with verdana, time_jis and arialuni fonts, the verdana font installed the t10 only and showed garbage twhen flipped till I added the t0 (actually there is on t0 extension just verdana.qpf).
time_jis als only installed the t10 version, however it shows okay in both orientations, but it only one size is available when flipped while in landscape mode has 6 sizes.
My experience with arialuni was exactly the same as yours, when I added the rotated font the garbage went away - hence my coment that yuo need both.
If you come up with a another wat I'd love to hear it as the arialuni fonts take up too much space
Stu