Sometimes your protag gives up the fight to be the most interesting person in the script.
We’ve all been there. That one supporting character who takes off, not coincidentally when the second act gets a little mushy.
It’s not okay for this to happen. It’s unfaithful. Protags sometimes sag under the weight of their goodness and rightness, their vanilla blandness. A sassy little piece on the side can make you feel better about yourself as a writer, but it’s wrong to cheat on your protag.
If you’re getting distracted by a fun minor character, you can end it with your boring protag and give the story to your new love, or you can patch it up with your protag by giving them some of that flash and complexity and sense of purpose that attracted you so much to your secondary character.