When using a tag in a binding expression it appears you also need to include the provider even it is the default.
This works:
numberFormat({[default]Line 1/Drives/MainExtruder/ActualSpeed}, "0.0") + " rpm"
but this does not:
numberFormat({Line 1/Drives/MainExtruder/ActualSpeed}, "0.0") + " rpm"
If it does need to be included then the “insert tag” button should add it.