For bird sitemaster …im sure there is a diplexer somewhere in your feeder-line and while your 900mhz measurement is ongoing on one sector the same sector on 1800mhz is transmitting hence there is a loop-back of rf which is greater than the value the bird sitemaster in-built filter can take hence the issue with your sitemaster OPEN result display even when the load is on the jumper…
Always power off the entire site if you want to do a complete feeder-line upto the antenna port using a load at the tower top.
If you are just taking the measurement from the jumper without removing the jumper all the way to the top no need to power up all sector.
Get an extra 50db load it would help in absorbing the excess coming off your transmitting module in a live environment.