Sorry, skimmed the post and took a look at the image. About half your engineers are "assisting" the missile silo, so I assumed that that was the issue.
Try selecting them all and hitting [END] on your keyboard. That will completely clear all orders (manual and auto-assigned) and make them pick new targets. Do note: a silo that is in the cooldown period between warheads (there's a several minute delay between building warheads of the same type) will still "attract" Engineering assistance, even though the engineers can't do anything (similar to the 3-second-post-damage repair delay: the engineer will be attempting to assist without being able to do anything). If you want to make sure the silo isn't attracting attention, put it into paused-building mode / stand-down.
But making them all re-evaluate their targets should make them properly split the workload again. If that doesn't work, there might be something buggy.