I recently upgraded from version 1.9.5, where I didn't see this issue. I have several FreeStyleBuilds that run on Swarm reviews. They execute concurrent builds, one per agent. They use the P4_CLIENT environment variable to find what files are unshelved in the default changelist, but when two builds start at similar times one of them will have the P4_CLIENT variable from the other.
My client names do not use the executor number because there is only ever one executor, so this is not a duplicate of
I have managed to reproduce the error with the attached build configuration (server credential name removed). It has a timer trigger for 1/minute but I had to spam the Build button with two agents connected to the master to get the job to fail.