When upgrading from 1.9.5 to 2.11.1, it seems that matrix builds are broken as far as the Perforce sync is concerned.

Effect of the upgrade: The plugin seems to cache and reuse the last workspace used for a given Perforce changelist. Although two different Perforce clients are defined for the matrix, a wrong client is therefore picked-up leading to the fatal "file not under client's root" message...

Workaround: downgrade to 1.9.5 to get back functional matrix builds