I am uncertain if this is a bug or just a rules interaction that one would not expect from a plain English reading of The Eternal Wanderer.
I had The Eternal Wanderer in play against my opponent's board of Huatli, Poet of Unity and Voracious Hydra. My opponent cast Doors of Durin and attacked me with Huatli and The Eternal Wanderer with Voracious Hydra (since they could not attack the planeswalker with both creatures due to its restriction of only allowing a single attacker against it). Doors of Durin triggered and my opponent put a Trumpeting Carnosaur into play tapped and attacking The Eternal Wanderer.
My expectation was that the Trumpeting Carnosaur would be unable to target The Eternal Wanderer with its attack since the Voracious Hydra was already attacking the planeswalker.
I had The Eternal Wanderer in play against my opponent's board of Huatli, Poet of Unity and Voracious Hydra. My opponent cast Doors of Durin and attacked me with Huatli and The Eternal Wanderer with Voracious Hydra (since they could not attack the planeswalker with both creatures due to its restriction of only allowing a single attacker against it). Doors of Durin triggered and my opponent put a Trumpeting Carnosaur into play tapped and attacking The Eternal Wanderer.
My expectation was that the Trumpeting Carnosaur would be unable to target The Eternal Wanderer with its attack since the Voracious Hydra was already attacking the planeswalker.