KG would know for sure, but I thought it was because of leftover air inside the tubes, no? Arcs are either because components are too close for the voltage they are trying to displace, or because there's still a conductor (I.E. air). I vote for the latter, since the tube tests normal. It will affect the life of the tube (I.E. shorter), but should work fine for now.
Someone confirm or deny, this is "thinking out loud" stuff -- which I liken to chewing with one's mouth open.