Originally Posted by
HerrKaLeun
That OEM rim tape is just to protect the inner tube from the spokes. it is not necessarily air-tight. Try some actual tubeless rim tape. it seems obvious that the air somehow escapes through the tapes and the spoke holes in that void between where that factory hole is.
Not necessarily...some tubeless wheelsets come pretaped with tubeless rim tape...my American Classic wheels did. Since the OP said the valves were included (and preinstalled even) I’d guess it’s tubeless rim tape.