Apparently, the VTK 9.4.0 public API exposes VTK’s internal copy of nlohmannjson (this wasn’t the case in VTK 9.3.1). If you built and installed VTK yourself, then you can reconfigure VTK to use the same nlohmannjson that you use in your app, and then re-install VTK. This should remove the conflict.
Try changing the following setting in VTK’s cmake cache, and then rebuild:
VTK_MODULE_USE_EXTERNAL_VTK_nlohmannjson=ON