Andrew
If your source is 1440 x 1080 HD anamorphic 16:9
When you set the project preset manually File Menu/New/Project, you set in Change Settings would look like
When you close out of there and get into the Expert workspace Edit Menu/Project Settings/General, the readings
should look like this
Based on what you said, probably OK to this point.
My most likely candidate for the cause of what you described is the choice of YouTube preset in Publish+Share/Social Websites/YouTube - maybe something going on in the YouTube conversion related to recognition of the 16:9 flag to stretch the 1440 x 1080 to 1920 x 1080. But, let us see what happens when you use the YouTube preset suggested.
PAL - High Definition Video for YouTube 1920 x 1080
Looking forward to the outcome.
Thanks.
ATR