So have a look at Preferences>Startup. What do you have selected for New Document?
Since your template already has viewports established for scenes in your model, there must be a SketchUp reference model included in the template. The correct way to use the template and get those existing viewports to show the scenes of your new model would be to replace the original SKP reference with the new reference file. So instead of Send to LayOut or File>Insert, you need to open the template, go to File>Document Setup, select the SKP reference and choose Relink. Choose the new SKP file and assuming your new model has scene names that exactly match the ones in your templates reference SKP, the viewports will display your new model.
Sending to LayOut and File>Insert would just add a new SKP file reference because with those methods of getting the file into LO, you aren’t doing anything to replace the original SKP file.
My template with the dummy SKP reference.
Click Relink, select desired SKP file. Done.
FWIW, this part is nothing new in LO2019. It’s worked that way from the very first version.