Hi WAG,
From the description of your issue I am assuming that you are either playing or creating a 'mission', but I may be wrong in that assumption. Personally I never use the 'mission' aspect of SSE, it is an online only game for me so I cannot comment on the 'mission' aspects.
In attempting to recreate your issue as far as I can, I created a multiplayer session which included the Inland dlc and used a separate pc to connect to the session.
The server, (the barge in these pictures) has:
Intel i3 4170 cpu running at 4.0Ghz
8Gb ram
Nvidia GTX-650Ti
Windows 7 Ultimate x64
SSE 1.5.5
Community patch 1.0
The player pc, (the Cratus in these pictures) has:
Intel i7 3930K cpu running at 4.6Ghz
16Gb ram
Nvidia GTX-980Ti x 2 in sli
Windows 7 Professional x64
SSE 1.5.5
Community patch 1.0
Community patch 1.3 - all of the barges
Picture 1 shows both Cratus and the Coal Barge with navigation lights on before attempting to connect a line.
Picture 2 shows the two vessels connected with a tow line and the navigation lights are still on.
Picture 3 shows the active connection on the Cratus.
I appreciate that a multiplayer environment may not actually replicate your situation but it’s the best I can do, and as you can see I was unable to recreate your issue. My results corroborate those obtained by Traddles but I am uncertain if Traddles results were obtained in a 'mission' or multiplayer environment. Unfortunately this is ultimately immaterial as none of this would appear to help you at all.