Any guesses out there ?

Hi, Dove. I know the "definition" of a Hybrid Tea is generally accepted as one bloom per stem, but roses are such a mixed bag of traits. It is VERY common for HTs to bloom in small to large clusters, just as it is for floribundas to provide one flower to the stem. Cluster flowering is pretty common in the several species represented in modern roses. Your cluster of blooms on Just Joey is well within the norm for not only that rose, but just about any other Hybrid Tea you could find for sale. It wouldn't be considered a sport unless the flowers were materially different from the others the plant usually produces, such as fewer or greater number of petals, different color, stripes, mottling, etc.; the growth noticably different as in elongated canes as in the case of a climber, or much more dwarf resembling a mini mutation, etc.; or something like a very fragrant rose regularly producing a scentless one, or vice versa. When the characteristic is one which is common within the range of traits possible, it's not a mutation, just an expression of what the thing CAN do. You may have never seen Just Joey do what it's doing for you at the moment, but it's a common thing to find on it, and most other Hybrid Teas.