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.
How did it work in previous versions? I do not use custom palettes except for CutContour for cutting and after X5 when color management was done I remember they were addressing color palettes in X6. One of the problems with color palettes at that time is that CorelDRAW was allowing commercial swatches to be renamed causing issues with conversion in output devices.
CorelDRAW was allowing several non-compliant mistakes to be made by the user previously and they were addressing them in X6 and the patches in X6 and I believe X7.
If you have 2018 to check the behavior against that would be a great start. I believe it forcing name changes was part of the process but with 2019 the process should duplicate 2018.
I'm not sure to understand you correctly, because you talk about "document palette" first, but later you talk about color style swatch, so it's relative to create a Color Styles
If you create a color style using a Pantone, and Publish as PDF it should be output as Pantone.
A mention about color styles was just a little offtopic but still it's a problem. It actually exports as spot color but inherits name from color styles swatch name, not from actual pantone. Will try to post screenshots tommorow when i'm at work.
Unfortunatelly i unistalled 2018 so i can't check this but i believe it was working properly. There was an issue with previous Draw versions, can't remember which ones.
I remember an issue with custom spot palettes not exporting to PDF if the auto update to the document palette was disabled.