Line breaks aren't going to work in the subject line of an email as that field is plain text by nature.
...I'd be willing to bet that it is also automatically removing extra spaces when you use field variables like you are doing.
Try this instead:
${number} | ${category} | ${subcategory}
...and it would be formatted with a space after the number, then a vertical line, then a space, then the category, then a space, then a line, then a space, then sub category.
For example = "INC0010234 | Email | Login Issue"
...if that works, you can add spaces in an attempt to "buffer" it more:
${number} | ${category} | ${subcategory}
