Hi Manfred,
thanks to “remind” me of that longish explanations / excursus 
.
The Export setting “Same as Soft Proofing” works together with Soft Proofing ON / the set colorspace.
As you indicated, you had exported your image to Nik as TIFF / DxO WideGamut,
which is this setting
(= the file gets converted to DxO WideGamut / not to mix up with DxO’s WCS)
After this file was/is processed by the Nik app and returned to PL
it was/is (@Egregius) still in DxO WideGamut
(have also checked this in PS)
.
The TIFF file processed in this way was/is your new “Source” and the Export with “Same as Soft Proofing” did/does not convert to the desired color space with SP OFF …
which means
- either activate SP/sRGB + export w/ Same as SP
- or export to sRGB
about SP → see …