Difference between specfem2d versions

[ConallEvans]

Hi,
I've been using specfem2d so simulate highly heterogeneous material for wave propagation. This has been to replicate work done by a colleague of mine. I found that with identical in-params, he and I have ended up with output velocity seismograms with a polarity flip of *-1. The version file says that we are using the same version. Is this something known and how can it be rectified?

For now, I am using a carbon copy of the software sent directly between us to preserve our workflow.
Thanks!