the CHGR0 is on the BCCH frequency, which is usually using a frequency plan a little cleaner than the CHGR1.
As a result, it is not surprising to have better QOS on CHGR0 than on CHGR1.
I’ve heard that we have noticed that on our network too, but I didn’t work on it myself.
What’s sure is that if there is a SDCCH ts on CHGR1, then it will be the one that will be used first. Once this SDCCH is full, then the one on CHGR0 will be used.
See if the SD Drop rate occurs for LU or for non-LU. If mostly for LU, then consider than MS are right at the cell border, therefore any slight change in interference will have a big impact.