I've seen several other people post this question in this forum, but none of them got an answer. I don't expect any better response, but I don't know what else to try.
When I create or edit a User Preset in the Export module, it exports the image correctly, but does not save the Preset correctly. I created a new Preset just now, changed the Export To to Hard Drive, specified a specific output folder, set the quality and resize settings, and checked the box to have it do output sharpening for the screen. It created the jpeg properly, and it saved a new preset file in the correct location. But the preset file doesn't have any of my settings in it. It's all the default values, such as Export to Email, size of 500 px, no sharpening, etc.
How do I fix this?
Well, I figured it out, so I thought I'd share the answer for the next time someone runs into this. It turned out to be the sequence of the steps that was the problem.
I was Adding the new User Preset, then setting the various settings the way I wanted them and then clicking Export. I assumed it would update the template and export the file, since there was no separate Save button. Wrong.
The correct, utterly counter-intuitive, sequence is to open the Export dialog, make all your desired settings, THEN click the Add button to create the new preset. You don't even have to click the Export button. Closing the Add Preset dialog after giving it a unique name causes LR to save the new preset.
So, there you go. I hope it helps someone.