I believe a lot of people are viewing Jayfeather's contrasting reaction to Brambleclaw and Leafpool's deaths through a very black-and-white lens.
Jayfeather still had a lot of unresolved feelings about Leafpool being his true mother at the time she died. Although personally painful to read, his emotions at Leafpool's vigil were realistic. The truth of his birth derailed Jayfeather's life as he knew it; this is not something that someone just "gets over" because the one that hurt him had pure intentions. And the truth hurt him on a slightly deeper level because Leafpool was also his mentor. She was his example of what a medicine cat should be.
Leafpool and Squirrelstar had the best of intentions for Jayfeather, Lionblaze, and Hollyleaf when they lied about their birth, BUT all three of those cats suffered deeply as a result of the truth coming out. Multiple things can be true at once!
A lot of us (rightfully) criticize Brambleclaw's character because of how withdrawn he became from Jayfeather and Lionblaze following the Gathering & Hollyleaf's disappearance in the tunnels. One thing I never personally considered before is the fact that at this point, the Clan believes that Hollyleaf is more than likely dead. Brambleclaw is not only struggling with the knowledge that the three kits he raised weren't his, but that one of them "died" before he as a father could receive any type of closure. That is a painful weight to carry on top of being the Clan's deputy. How he treats Jayfeather and Lionblaze after the events of Sunrise is a valid critique but through Jayfeather's eyes, Brambleclaw is still his father. It would make sense that Jayfeather's grief for Brambleclaw is more palpable and much more raw on the surface because it doesn't carry as much complication as his grief for Leafpool did.
Some people forget entirely that Jayfeather did grieve Leafpool, and quite deeply. He poured himself into his work as a medicine cat to avoid speaking with anybody about his grief for Leafpool. He avoided going to the half-moon meetings at the Moonpool because he wasn't ready to see his mother yet. The other medicine cats literally had to have an intervention for him. 😭
It’s a shame that the bonus scene where Jayfeather talks with Leafpool in StarClan is a Barnes and Nobel exclusive, so all of the international audience is excluded from it unless we find a version of it online. It’s such a great and important bit of content.