To clarify, this is happening to all heli doors in the mod and was not happening with DayZ 1.27

I found a resolution to this issue

Here was the issue/fix for posterity:

A design decision in my Heli mod was to make heli doors non-removable, and auto spawned onto the heli when a door is missing.
The action to attach the doors is tied to OnUpdate and a toggle field when the heli is first spawned or a door is removed/deleted.
The action to attach the doors was not limited in context, so would happen on both Server and Client side.
Since 1.28, the above error and script crash started happening.
I limited the door attachment to only run in the Server context and the error and script crash went away.

How this relates to the Client side error that was happening when a heli came into the player's network bubble is beyond my understanding.

This ticket can now be closed.