Well, it turns out I wasn't crazy. It's been christened Voodood Darkening (I think by Julie Beck who put the test together). It felt like voodoo. It turns out in thin layers titanium white tends to go a little darker as it dries. Since I tend paint in thin layers until the end, no wonder I was struggling. This video shows that this person learnt it isn't just happening in organic pigments such as umbers - but in phthalo blue and cad red deep. I can add other blues such as Cobalt to that list based on my experience. The cause isn't understood.
The remedy appears to be thicker layers. A little bodied oil such as stand oil can also address it. However, considering the 'fat over lean' rule - which is really about painting thinner layers over a layer that hasn't fully cured - it might be something we put up at times with in early stages. Then really lay down those highlights and light sections with some generous paint.