Forget about the print resolution it not what image quality is about. You degraded your template from a 7.36MP images to a 1,77MP image You have few pixels to store to composite details in the 1920x1080 canvas. Print Resolution ppi is print pixel size 72ppi pixels are 1/72" by 1/72" 300ppi pixels are 1/300" by 1/300".
You can store more image detail in a 3200px by 2300px document the tor can store in a 1920px by 1080px document you you also changed the template Aspect Ratio. 1920x1080 is a 16:9 Aspect ratio. 3200x2300 is a 32:23 Aspect Ratio. You can not interpolate images without loosing image quality and it not just about the number of pixels. Its the number of high quality pixels. If you have a 8MP out of focus image you have 8MP poor quality pixels where with an in focus image you will have 8MP High quality pixels. It you interpolate thoes 8MP images to a 1.77MP image you gabs to through away some of the image details you have. You no longer will have your 8MP Image you have a new smaller image with 1.77MP interpolated from the 8MP images the image quality will be poorer.