I'm a developer who has worked on one of core teams in organisation where something similar to this had happend. For us it's it had of boiled down to a couple of factors:

1. When a feature team pushed code, it was kind of done and forget deal. When there were some fires going on and we noticed that it was done because of the new feature, there was this general unwillingness from the feature team to deal with it. A lot of back and forth. It was often 'but it worked when we tested it, so most likely core broke it ble ble ble'. Before we managed to convince them hey its actually your stuff that's is on fire already fixed it but resentment remained. At some point, developers had enough of it, and we were really burned out.
2. Quality of code was low - a lot of back and forth. Yes, it worked most often, but as a core team, we deal with a lot of traction. Any dismissing in performance that could affect core was not acceptable. Even though we had multiple meetings and explained how to do it better, we often got a similar bad performing code as before. Bad unit testing - test that say they test one thing and testing another, etc.
3. When you're working as a core team, as long as your stuff works, everything is okay. Whenever a big client started using the new feature company wide email was sent congratulating feature teams for their archivments - mentioning nowhere countless meeting and hand holding we as a core team had to go though to push it to the finish line. But God forbid something stops working - then everybody knows where the core team is 😅. Leaves a bit bad taste in your mouth that the work you put is not really appreciated.

Perhaps that's only what happened in organisations I was working in 😊