When opening 32-bit PSD or PSB files the pixel layer masks of 32-bit layers are not imported properly. It is just the mask that is not imported correctly, the RGB channels are correct.
Attached is a very simple PSD file saved form Photoshop CC 2017 (A red pixel layer with a circular shaped pixel mask). The attached screenshot shows how the file should look like on the left (Photoshop) and how it is imported by Affinity on the right.
This is likely a general PSD/PSB compatibility issue, not Photoshop version or image specific as I also reproduced this using a different PSD exporter.
Affinity Photo 1.5.0.43 (Beta) was used.
Let me know if a fix is available and I'm happy to verify if this is working correctly.
Daniel
affinity_32bit_mask.psd