VWR-13868 has been fixed with the mandatory viewer update.
Release Notes for Second Life 1.23(5) Release Candidate – October 15, 2009
Changes
* VWR-13868[c]: Sculpted prims in complex linked sets vanish – Raised ‘RENDERMAXNODESIZE’ from 4096 to 8192
VWR-13868 is the defect report for when you go to zoom in on something half the prims [...]
