I think you might be right. Many thanks I must have changed what I do without realising. Regards.
You probably select download as gpx type route ? Try select download as gpx type track.
When opening a route from my cycle club (ie ID: 478401) on my laptop (windows 10) it is fine @ 49.3mi / 79.4km. When then downloaded as a GPX & copied into my Garmin it becomes 45.9mi / 73.8Km and the route has been somplified & straightened.
Looking at the map in my Garmin the route doesn't folow the roads but it just a straight line.
If I load the route from the my laptop back into Plotaroute its the same simplified shortened route with straight lines. This never happened & only seems to have been doing this for the past number of months. Why?
If I us TCX (Which I don't normally) things seem to be OK re file size etc - but I've not tested this yet to navigate.