My personal guess is that it’s variable failure (probably caused by the mirrors). Messages get through sometimes and don’t get through other times. The “operates perfectly” situation may be taking two or three tries to complete , but you don’t realize it. The repair utility has a higher standard.
But that’s just a guess.
It would be interesting to cover the mirrors with sheets and see if that affects the repair, but I know that’s a lot of work for what may be very little information. 
If it’s working now for what you want since you added the additional repeater , you may just not want to worry about it.