Losing Turn Speed on Ship Mod

SailSurfer

During a mod, I found that, having a bonus to turn speed, I lost 3 points of turn speed instead.  After a rather lengthy discussion in chat, others have reported the same issue, all on turn speed.  At worst, a zero or +1 would be the result.  A negative result should not be possible.

I used my sb rank of 20 at the Seville Ship Master on a Custom Heavy Frigate (Cove ship), using a Medium Storm Sail, Modified Standard Gun Port, Admiral's Cabin, and an Infirmary.