Thank you for your reply.
About your questions, here are some answers:
1/ Which ALU release are you using ?
We have UA6 release
2/ Collect your 2G to 3G reselection attempts and successes counters (from 3G network, not 2G), and compare those amouns before vs after
By doing the comparaison, I can see that there is a decrease of both attempts and succes.
3/ Check this 2G3G reselection success rate (attempt / succes) between before and after.
The success rate is stable (no significative change).
We did an upgrade of Huawei UMG from core network side and just after the reselection issue occurs.