I've been having a similar issue in Publisher.
I am a sign designer, meaning that I show sign designs at varying scales on standard-sized paper sheets (usually Tabloid, or 11"x17" - I'm in the USA), with dimensions to show how big the signs are. I am still pretty new to the Affinity suite, trying to transition from Adobe Illustrator. I've been using Designer to lay out the sign designs, but I'm using Publisher to set up my pages for presentation, using the Master page to set up the title block that repeats across all pages. Then on each page I place a link (or multiple links) to my Designer document, and use the Vector Crop tool as necessary to show the various views.
Because I need to work at different scales depending on the size of the sign, I set up multiple artboards in Designer, then use the drop-down list to select the artboard with the design I want to show for each placed item in Publisher. I have Publisher set to update the links automatically so that any changes I make in Designer are reflected in my Publisher document.
I've been saving all of my work in Google Drive, and sometimes it loses connection when my Mac wakes from sleep - I was suspecting that this may be the issue. But adding artboards to the Designer document after the Publisher document has been set up could be the culprit - this morning I was working on a project where the client wants to add some additional sign types, so I added artboards in Designer for them, then opened Publisher to find all of my placed links show the entire Designer document rather than the individual artboards, which means I have to go back through all of the pages to edit the placed files to show only individual artboards, and then tweak the size and placement of each, causing me spend time to redo work that was already done.