Colour Variation when using "Darken" transparency

[Pre-Press]

Why am I getting a colour variation on screen when using the "Darken" mode in the transparency field.
See attached screenshot. 
The "Substrate" spot colour has the "Darken" applied which is layered on top of the colour wedges.
Should the colours of the wedges not display the same irrespective of whether "Overprint Preview" is on or off?
P.S.
I've tried the "Multiply" function as well but there is still a variation in colour albeit not as bad as the "Darken".

[Ton Frederiks]

I suppose the underlying colors are a neutral grey? I cannot reproduce that, there is a slight variation with Overprint on and off, but not a color shift.

Can you share an example file?

[c.pfaffenbichler]

When working with Spot Colors one should stick, in my opinion, to managing Overprinting with »Overprint …« (in the Attributes Panel) instead of using Blend Modes. 

[Ton Frederiks]

Darken separates the spot in the same way as Overprint and has the advantage that pdf viewers that don't support Overprint (like Apple Preview and the preview in this forum) show the expected preview. Darken also gives a better result when the spot has to be converted to process colors.

[Pre-Press]

Thanks for the response Ton.

Your reasoning is correct.
As a Prepress house, our Pdf files we send out for approval are Press ready files, meaning that they are colour separated correctly. We have dozens of clients with different viewing software, thus creating the Preview dilemma you mentioned. We have found in the past that the "Darken" mode worked well for us up until recently when I encountered the colour variation as mentioned in my intial enquiry.
Find attached a sample illustrator file showing the difference between Overprint, Darken and Multiply with Spot colours.

P.S.
We avoid "Multiply" because it changes the values of the underlying colours.

[Ton Frederiks]

Good to hear the reason for Darken mode.

Unfortunately I  see no sample file attached (.ai files don't work in this forum, sometimes pdf files do).

Links to Dropbox, WeTransfer, CC filesharing, Google are options

[Pre-Press]

Hi Ton.

I've attached an illustrator Pdf now.

Cheers.

[Ton Frederiks]

Thanks for sharing, it is indeed a preview problem.

The file separates fine and when Overprint preview is turned on the preview looks fine.

With Overprint Preview off the preview problem becomes visible.

It is not only Illustrator, in Acrobat the file looks fine when Overprint Preview is active but when it is turned off in the preferences, it has the same problem as Illustrator. So at least it is consistent.

It seems to be shared preview code between multiple Adobe applications (InDesign included).

I am not sure where to report this, the Acrobat Uservoice or Illustrator Uservoice, maybe both:

https://acrobat.uservoice.com

https://illustrator.uservoice.com

[Pre-Press]

Thanks for your help Ton.
I'll progress this issue with the illustrator team now.

[Ton Frederiks]

Thanks, let us know where to vote.

[Pre-Press]

https://illustrator.uservoice.com/forums/601447-illustrator-desktop-bugs/suggestions/44561601-colour...

[Pre-Press]

Thank you for your response.
Ton's scenario below is exactly why we have resorted to using the transparency functionality rather than Overprint.