Print Preview Scale keeps reverting to odd value

Enjoying LC and learning heaps but I seem to have an issue with the Scale in Print Preview.  I have set "Current Drawing Preferences" to A2 Portrait - I need to print out a rectangular template 450 x 300 mm.  When I went to the print shop and got it printed (PDF) I noticed the physical dimensions were not correct.  The issue was that the scaling reverted back to 1:13294 which would make the hard printed copy slightly smaller than intended.  It needs to be exactly right as it is for a house window.

So you can manually change the scale to 1:1 and then export to PDF, and if you print it out it works perfectly, however it doesn't seem to save this setting, even if you lock it.  So if you decide to make a change to the drawing a few days later (ie. reopen the file) and forget to change the scale back to 1:1 before exporting then you will have a problem with the physical print.

As you can see below there is more than enough room for the drawing to fit inside A2 format.  So, I am happy to keep manually checking it first before exporting, but it would be nice to be able to set this scaling properly to 1:1 so it doesn't change.  Any ideas?  Thanks.