Tony- I don't believe it is a deficiency for the reason I already gave. Not saying I'm right, just throwing the logic.
The commercial nurseries typically go quite heavy on the feeding. Ok, then, let's say it is a deficiency. Then, under your care with careful and regular fertilizing regime, things should have turned around but you say the plants still have issues.
Logically, the newly developed portion under your care should look problem free. Right?
It is a potassium deficiency, as a very certain fact... and to know it just by looking at a photo, because that's my job, for gigantic large scale nurseries and pot plant labs...
The same as Miltoniopsis that have a orange rot at the base of the growths and the bulbs. It is not a disease, but a deficiency...
I know as well the symptoms when a multi bloom and the leaf tips start to die off, and the exact reasons. I paid more than enough mineral analysis to know a little bit about the subject.
For the feeding, Hawaii and Taiwan tend to use a very high nitrogen/urea content fertilizer so the plants do grow big quickly ( we do the same for pot plant Phalaenopsis in 12cm...). They do not correct the pH on top of that, so the plant grows, but at a low pH the potassium is not properly absorbed by multifloral and some barbata Paphiopedilum, then you have 2 reasons for the deficiency.
As for when a plant goes down, well, long story. But let's say that with some deficiencies, the plants will take years to recover, if they are able to. It induces some metabolic changes that are very hard to revert. Potassium in Paphiopedilum especially is not really mobile du to the plant structure. There is an equilibrium depending on the growth placement, the leaf age, and the roots ( where and on which growth), and once this equilibrium is broken, it can take ages, if ever, to get back the plant on track.
I always have a lot of fun about people talking about orchid nutrition. Most of them have absolutely no data to support their claims, other than 'well it works better with that fertilizer, so it is good, and here is the reason,different NPK'.... Nutrition science for orchids is much, much more complicated than just a direct test, less potassium and the plant grows, or this kind of things. There are massive interactions in terms of ion uptake, residual pH, combined ions needs, etc... that are known only in the industrial pot plant world, for sure.
Out of the 4 largest laboratories doing Phalaenopsis tissue culture ( market of 180 millions plants a year only in Europe), 3 for sure use an ammonium and urea based media, no nitrate. There is a reason for that... but you won't find any hobbyist-like or university basic level literature mentioning those media, and how they work.
There is a final point, as well. When a plant has a deficiency or toxicity, it is very difficult to get it to recover. In some cases it takes months. In many cases, years or more.... and in the orchid world, deficiencies are more common than toxicities by far.