Hello Agisoft,
Recently I had a couple of Sequoia-datasets (using v1.4.1 Sequoia firmware...) with partially missing channel-files, e.g. a single xxxGRE.TIF or xxxRED.TIF was missing. Typically only 1 single file from 400 is missing.
Unfortunately when Photoscan v1.3.x detects a missing channel, it treats all images as normal dataset and the multispectral-import does not appear.
I would suggest the following approach:
Please only check if the single multispectral image-tuple is complete (_GRE.TIF,_NIR.TIF,_RED.TIF,_REG.TIF (don't care about the rgb.JPG)). If one of the images in a 4-tuple is missing, skip only this tuple and continue searching.