If you summon a villager with the command

/summon Villager ~ ~ ~3

{Invulnerable:1,CustomName:Tim,Profession:2,Career:1,NoAI:1,CareerLevel:4}

and summon a lightning with

/summon LightningBolt

the villager gets morphed into a witch, and also loses the invulnerability.

This is a problem, since we use custom villagers as traders on our server, and with thunderstorms we lose them from time to time.

I would rather expect the "Invulnerability" flag to protect them not only from damage, but also from the morph-effect from lightning strikes.
I understand, that the current state is "the morphing of an entity is not prevented by the invulnerable flag, because the lightning does not call the damage trigger event in the code".

If this is WAI, this may be closed, but I found no source confirming that invulnerability is not MEANT to provide immunity against the effects of lightning strike.