I agree that it is probably a rootstock tree. Cleopatra (often called Cleo for short) is the only tangerine that has been used frequently as a rootstock to my knowledge, but I would discribe it as sour more than bitter. Calamondin is a hardy, sour tangerine like fruit that has often been planted as an ornamental and is used for baking, drinks, and marmalade. There are also a couple other rootstock trees that might be mistaken for tangerines like Carrizo citrange which has a small bright orange fruit and trifoliate leaves. Pictures would be helpful.