TestStand and LVLIP are always a bit special. I found out that all visible controls on the called VI from LVLIBP must be in public scope.
Did you compile the code as debug? If so, this rule partially applies to subVI's as well.
Sometimes it is "Disabled structures" with broken code that cause problems.
=> See my checklist:
https://forums.ni.com/t5/DQMH-Consortium-Toolkits/my-DQMH-PPL-and-VILIB-PPL-best-practices/td-p/4155993
Claude