Evra, Halcyon Witness corner case

BlessedOne

This is a corner case that would be hard to duplicate.
I played a round of training grounds using Angrath.

I ended up with two Evras on my board.

I used The Eldest Reborn to pull one from my opponents graveyard.
Later in the game I used Angrath's second ability, Captain's Command, to pull another Evra into my field.
The commanded, temporary Evra was at the top of the stack in the fourth position.
The permanent Evra was in the third position.
For a moment I had four creatures on the field.

The interaction of the two Evras was the issue.

The permanent Evra (brought in through Eldest Reborn) was being buffed by the second, temporary, Evra.
The temporary Evra was showing the obvious animation of buffing the permanent Evra... the arc that travels from one creature to another that is being buffed.
The temporary Evra did one buff animation and was done.

All of this happened during the attack phase.
The permanent Evra attacked.
The permanent Evra's stats would go up and down from +9/+9 to +17/+17.
It did this for a while, less than a minute. It caused concern because I wasn't expecting this. Was it going to stop?
Then it did...

Then the temporary Evra attacked...
The permanent Evra was buffed over and over again from +9/+9 to +17/+17.
It lasted a good long while also.

The permanent Evra's attack had already resolved.
Why was the temporary Evra buffing the permanent Evra during its attack phase?

Finally, the attack resolved, and the temporary Evra died.

The game resumed normally.