Trying to define a user variable for the export path is beyond broken. Creating the first variable only allows you to enter 1 character for the name and 1 character for the value. Creating a second variable allows you to enter 2 characters for the name, 1 character for the value..etc.
Affinity Designer 1.7.1
https://www.youtube.com/watch?v=YsyriBhvq6s