I only have access to a 10.5 server. Your workbook behaves correctly there.
I assume you encountered this in your own workbook, and then hacked up the equivalent Superstore example that you uploaded here. If so, that shows a clear consistency that is not specific to the workbook.
If you don't get other responses, you could file a support ticket. The upgrade from 10.5 to 2018.1 seems to have introduced unexpected behavior.
Trying to think of what else would add info to understanding this problem... What happens if you publish something like Dashboard 2 in the attached? (And then maybe introduce the swap filter to one, and then turn it on or off...)
I'm not saying this is a workaround (because it's not!) Just a way to add more data points to what might be happening in there.
Publishing your workbook shows all 3 sheets. After introducing the sheet swap filter to only one sheet and setting the default parameter value set to not display that sheet, and then switching the parameter to show that sheet, results in the same error. Looks like custom legend shapes are only appearing for the default sheet set by sheet swap. Agree that there is some unexpected behavior with the upgrade.
If I were in your place, I would file a support ticket with Tableau. For the record, I don't work for Tableau, so all I can do is suggest that you do this.
Do you know how/where to file a support case?
Yes. Thanks for the tips though!