A curious thing I've seen a few stories do is that after a long time following the main story, like at least two arcs, it will suddenly cut to doing an extended backstory origin for the main character.
And every time I just wonder... why now? Why not closer to the start? Why interrupt the main story I'm invested in at this point to tell me stuff I don't feel like I need to know any longer? I already know what's gonna happen based on what you've shown so far.
I wanted to see where the story was going next. Not an extended play by play of the backstory. And none of my favourite characters are here now.
Honestly my main issue is more about how long the section is, rather than it being backstory. Please keep it more succinct, so we can go back to the plot. Otherwise I might lose interest before you're done.
In a similar vein I've had issues with stories that go on lengthy sidequests without the main character present. One I'm reading right now has been on this sort of sidequest for a while. I keep waiting for the main character to come onto the scene, and with every chapter he keeps not showing up.
I'm not even sure if any of the characters there are ones I've seen before. The slow release schedule of chapters means I've forgotten if they're previously established, or new. Re-reading everything so far would probably give me the answers, but I don't quite care enough to do that.
I am not opposed to showing alternate perspectives in stories. I quite like seeing how others react to the main character(s), and what they're doing. The key thing is that those parts are kept short.