That's not a plot hole, I mean it would be a plot holes if the story said said they were complete strangers, but the fact they met as children was just not apart of the plot, they were children so I doubt they would remember that, it's not really a hole just a lack of depth that you wanted, plot holes are things that contradict what may have been showed before, but I think the author put that there to show the two have a deep connection as even though they might vnit remember but they have met before
Okay (kinda) plot hole for the backstory because THEY FREAKIN MET BEFORE
But this is the first story with animal characteristics without 4 ears (HAHAHAHAAHHA) I've read