A "jump" down after a reboot may just be a metering error that is fixing itself. I would think a steady (but faster than normal) decline in battery level to possibly indicate a failing battery. But a vertical (or nearly so) is probably just a metering error. I've seen this happen once or twice on my phone, and everything seems fine with the battery. It just seems like a fluke.
As mentioned, battery metering is far from accurate. It shouldn't be taken for granted that 1% increments, or even close to that are accurate.
Battery life (and whether yours is failing or not) is a very difficult thing to compare, as it depends greatly on how you use your phone, and reception in your area. But the drainage you described, for watching YouTube on LTE (pretty power intensive activity) seems pretty normal to me.
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Phone: HTC One X (AT&T version) Rooted, Bootloader Unlocked, S-Off
Tablet: Samsung Galaxy Note 8 (WiFi only) Rooted