Just bumped into this (once again). If you create a custom spot swatch in your document palette by changing its mode from process to spot it won't export as spot unless you change it's name first, then change property to spot. If you do it other way around it won't work. It was a bug in previous versions that i believe was resolved in 2018 or 2017 and now it's back again. I've also noticed that if you create color style swatch with pantone it will export named after swatch name (for example „ColorStyle1”) instead of pantone name.
OK, my bad :) I remember that problem was occuring when i fresh installed 2019 when it came out, now (after updates) it seems to be working properly, Pantone names are preserved if using Color Style swatched.
I always have auto update palette set to off. Tried to find out if it changes anything if i turn it on. No luck.
Since I always manufacture or print something I never use anything but commercial palettes and the document palette. The document palette update was an old bug and I never tested it in the last version or two just left auto update on.