I use on Windows10 and macOS 10.15.1 and the latest versions of affinity designer.
when I export a DIN A5 Artboard to a print file in 300 dpi, the size of the document will be showed later in the properties of the pdf-Viewer more than the size in affinity designer.
for example: I create the DIN A5 artboard in the size 210 mm x 148 mm. in the properties of the file document I give 1mm around all sides for the cutting in the printing factory. so I must have a total size from 150 mm x 212 mm. BUT equal with which System (Windows or macOS) I export the document file, I became after opening the file with macOS a total size from 150,1 mm x 212,1 mm. when I open the same file with Windows 10, the size is correct.
when I want to load up the file from macOS to a printing company, they don't accept the file (wrong size). from a windows system it works!!??
I don't know, where the mistake is...
two screenshots for bettet understanding...
best regards