Hi had this with my 2014 T5.1 BiTdi, and drive me mad .. just couldn't work it out due to how sporadic it was. But definitely warm engine thing, never a cold engine.
In the end I traded in the van for T6 150 (never going BiT again). But here is my version of issues (probably what you may face down the road, hopefully it might help some way).
For me the plume at pull off/after slow corners was a grey colour, but slowly that went a more blue colour.
Grey plume started happening around 90k miles on the clock, I'd notice it once every month or so, but by 100k miles it was blue, saw it weekly when engine was warmed up.
Van had started to use a tad of oil from around 60k, but not much. By 100k, it was using a load of oil, but had an oil leak as well... so was hard to say how much was real!
I was started to get dash warning for DPF, but that would seemingly would self-sort itself out. But also started to get dash warning for Oil Viscosity ... which I beleive was dpf back pressure! DPF becoming more blocked, back pressure to engine, fuel not being burned correctly and depositing in the oil. Change the oil asap! That viscosity problem went, but only for 4 or 5 months.
I also suspect was this clogging up the dpf, then getting burned off when engine is hot??
However the van still drove like a dream! Such a lovely drive (far better than my newer T6 150!)
Personally (I'm not a mechanic by the way!) I think it's perfect storm of many issues. I think injectors could have been issue (I did once have injector fault found, but never got dash warning for it). I've also thought about a turbo oil leak issue (possibly the smaller turbo that I assume is used more a lower speeds (bad science alert!).
But either way, my wife got fed up with it (and my constant whinging), we lost faith in it which was a shame as we'd had the van for 8 cracking years! Sadly my old former trusty friend had to go (and cheaply

), and to someone who hopefully had the expertise, time, funds to fix it.
I hope you get to the bottom of it.
I agree with all the comments above; get Millers oil test, then compression test. You'll then know whether to stick or twist. Good luck!