Vining Paph?
 

The plant on the left is Paphiopedilum Living Desert x tonsum. The plant on the right is Paphiopedilum (Makuli x Ciliolare) x Cicada's Song. Both came from Carter & Holmes and seem to be growing well in S/H. Almost all my non-multifloral Paphs are growing together on this one shady shelf.

But, the Paph. Living Desert x tonsum makes substantial vertical growth with each new leaf. None of my other Paphs does this. Is this something from one of its ancestors? Did I just get a weird one? Does this happen with Paphs in insufficient light?

Yes, paphs can get leggy in insufficient light...

It isn't just a case of insufficient light, though. Many of the mottled leaf Maudiae-type Paphs have some predisposition to do this.

It may be an adaptation for growing among rapidly accumulating leaf litter or scrambling among rocks. It certainly varies from plant to plant even within a species or cross. Some may do it most of the time, some only in specific circumstances, some almost never.

This cross is 1/2 Paph tonsum and 1/4 Paph masterisanum, so this isn't a typical callosum/lawrenceanum/sukhakulii hybrid. Both of those species may live in pockets of humus in rocky environments. Paph tonsum in particular prefers a bit more light than most.

I've observed several environmental conditions that may promote this growth habit. Yes, low light - but also crowding, very high humidity, media gone bad. This particular plant may not be entirely happy in the s/h even if it generally grows well.

Especially because this is a Paph tonsum hybrid, higher light for this plant is the first thing to try. Good air circulation and avoid crowding it. But it may always tend toward this leggy habit. If later growths are shorter you could cut the leggy growth and root it as a cutting. You may see root stubs develop along it even well above the media.

I'll give it more light. Phals do bloom on those shelves. I don't think excess humidity is the issue; it's about 60% - 80% day - night this time of year. It's had this leggy tendency since I got it. I may try to make a top cutting later just so I can say I did it.