CA traffic volume is decreased by 2% after expansion of 1800 MHz band from 10MHz to 15MHz, 2nd Carrier is band 20 (800 MHz), Is this any UE capability issue for 15MHz 1800 band & 10 MHz 800 band in CA?
Since now you have more capacity on 1800 and likely with a better SINR/CQI than 800, the scheduler could be preferring to schedule data on 1800 than 800… it is not a problem if this is the only metric which has presented reduction.
Check de cluster total traffic, the total user throughput (considering CA and nonCA), etc…
Total traffic/DL user throughput/CA throughput all are improved only CA Traffic has been decreased after expansion.
Can you check Scell configuration & activation attempts if decreased?