I expanded a 6 wide RaidZ2 vdev to 10 wide using 16tb drives. I’ve found the same problem, zpool list -v shows available space increasing after running the inplace balancing script (I watched the free space increase whilst it was taking place, and the capacity used decrease but the dashboard widget never changed (96 tb capacity reported in widget, zpool reported 117 tb capacity).
Watching with interest, if there is any information I can offer happy to help.