Not sure why the shell length is affecting it. I haven't ran any shells through the tube yet, but I have replaced the followers before with the steel upgrades.@Red Squirrel 556 - does the tube magazine spring follower work well in that 870? I have one and it often fails to follow correctly when loaded with longer shells. Someone said it was plastic and to replace it with a steel one, but I've not taken it apart yet.
I've been an 870 guy for decades, and this is the only one that's had a defect. (it's same model as yours)
Could just have a bad spot in the tube or something. Take it down and check it with a light at the other end.