Printing envelopes in Pages on iPad - print scaling to Letter size paper

I’m trying to print envelopes on my iPad Pro using Pages, I can configure the size of the envelope on the screen, but when I print it always scales the output to full Letter size. The only way I have found around it is to place the text on the screen as though I’m printing to Letter size, and then load envelopes. But this only works with an older printer, my newer printer rejects it as being the wrong paper size!! Is this a bug in Pages? Or is there a setting somewhere to change this?