I believe this only holds for simpler cases like:
Quantity[1, "Meters"/"Seconds"] Quantity[2, "Seconds"]
I think it is confused by the ElectronVolts. If you change line 1 to:
v = 10 Quantity[1, "Electronvolts"] // UnitConvert
it also works automatically. An explicit UnitConvert is sometimes necessary I found out. I'm not sure if it should be considered as a bug or as a lack of implementation. N.b. If you don't use the second argument "SI" it will do the "SIBase" by default, works most of the times for me.