I definitely agree with the analysis that their animosity comes primarily from their point of views. Regardless of what happened between them, just who they are and how they view the world was bound to cause conflict between them. However there are two points that I want to make.
Vinanna was not Easthies's Childhood Master
He's her Apprentice to be the next Wise, like she invites Luluci to be in chapter 97.
Easthies's actual childhood master is this person we see in chapter 76, who is not a Knight at all. They have a similar haircut to Vinanna so initially I thought it could be her, but we have her in flashbacks in chapter 36 and 89 right in the same time period where she's in her Knight uniform, so her not wearing it in this scene in 76 doesn't make sense. I think this has to be someone else that's not a knight. If it was some sort of alternate uniform for Knights you think all the kid trainees would have identical ones.
Easthies continues to be this person's apprentice through at least the third test, he shows up in the Olly and Qifrey adventure montage in chapter 92 in this hat and robe set. It's possible there's an Officer/Enlisted divide in the Knights and Officers like Easthies have to do ordinary witch training through the 3rd/4th test but we just don't have that information to make a call. It's also possible something happened to his master that made him choose to quit his original path and become a knight.
The first time we see him as a knight is in Luluci's flashback, where he's with Vinanna while she's still a Knight and not Wise yet (check her hat on her desk).
Like everyone else in fandom, I assumed Easthies and Qifrey were forced to see each other a lot as kids because of Beldaruit and Vinanna's friendship, but after 97 it seems like that might not be true and it's entirely possible they didn't see each other much until they were teens.
2. Easthies Singles Qifrey Out to Help on Silver Eve
Though their animosity makes sense without there being an inciting incident, this interaction in the Silver Eve arc seems very weird without them having had a significant shared event in the past. Easthies singles Qifrey out to give him a sealed scroll in case something goes wrong. From chapter 50.
Qifrey uses it against Engeldale and explains what it is in chapter 69.
This interaction bugs me so much because I don't know what to make of it. Easthies might be assuming that Qifrey could be a target because of his connection to Beldaruit and is just trying to make sure Beldaruit doesn't have an obvious weakness to exploit but it's so weird. What a weird way to go about it as opposed to just, sending someone undercover to follow him. "I hate you, here's a shotgun." What the hell.
I love chapter 97 but it also throws pretty much everything we thought we knew about Easthies's past out the window.