i think i have resolved this via email, adding the resolution here just to close the question
your main template is using chrome-pdf, but the page you are appending is using the phantom-pdf recipe, which does not support flexbox and also has other default for the size of pdf page (it is A4 by default), the header/footer you have uses chrome-pdf recipe which has another different page size (i think it is size of Letter page as default in chrome), so when you merge page from chrome with page from phantom-pdf you see that mismatch in the end result.
if I just change the recipe of your inventory-signature.htm to chrome-pdf everything works as expected. (because chrome supports flexbox and it produces the same pdf page size as your other pages)