So no magic error, stupid mistake as usual: My app.config had two binding redirections: This was in the top of the file:
``` <dependentAssembly> <assemblyIdentity name="System.ValueTuple" publicKeyToken="cc7b13ffcd2ddd51" culture="neutral" /> <bindingRedirect oldVersion="0.0.0.0-4.0.2.0" newVersion="4.0.2.0" /> </dependentAssembly> ```
And the correct was on the bottom, as the file is few screens large I missed it, the first one was used by app.

ok this feels completely wrong but I cut
<dependentAssembly> <assemblyIdentity name="System.ValueTuple" publicKeyToken="cc7b13ffcd2ddd51" culture="neutral" /> <bindingRedirect oldVersion="0.0.0.0-4.0.3.0" newVersion="4.0.3.0" /> </dependentAssembly>
This out of my web.config for the main application.
I was really just seeing what happened to see if there was an underlying dependency or something, not expecting it to run. It just carried on working, all the new functions I have added in the last few days still work.