I often find that when working with luminous surfaces, if you are looking to demonstrate colour effects and therefore utilise the Source Colour functions with your luminaire definition, there is a great danger of later conversion of that photometry into Colour Mixer figures. When that happens, I believe that it can have a noticeably detrimental effect on the output (due to SRE being invoked with the Colour Mixer).
For instances, if you define a luminaire and give it a Source Colour, if you exit the dialog box and re-enter it there is no problem. If you click on the 'Luminous' picker in the Render Mode symbol area you'll still see that it's pointing to the chosen Source Colour.
However, if you define a second luminaire, you'll note that the system now defaults to the Colour Mixer tab of the dialog box. Given that most users probably don't use the CCT function in most designs, I'd bet that most people ignore this process in the luminaire definition steps. If you're not interested in this newly defined luminaire demonstrating CCT effects in the render, you could find that you don't get full output in the design.
Even more misleading is the fact that if you then use this luminaire in another design via 'Import AGi32' processes, the CCT figure converts to Colour Mixer (or at least it used to - I haven't done this recently, but it used to and I've never seen this come up as a bug fix). My guess is that if you add a luminaire to a collection and it's been defined with a Source Colour, it will probably also get affected by this same issue.
I think that I may not have uncovered all the ways that this issue could potentially affect a user, but if you investigate it and find out where the conversion occurs you'll probably get a better idea of the full ramifications of the problem.
Why was this not fixed in the major release this week?
This is a major bug in the program, one that can lead to very erroneous results.
I believe you should remove this whole functionality until you can be sure that people cannot end up with severely downgraded output from their fixtures due to the program changing fundamental properties of their defined luminaires.