Hello Murray,
I think ImageMagick or similar application should allow to remove the alpha mark from the image EXIF. If you check the EXIF tags using, for example, Exiftool utility, you'll see that the fourth "extra" sample is marked as Unassociated Alpha. And also if I open the image in QGIS and switch the representation to singleband display, the fourth band is also marked as alpha.
Here's also a sample script for PhotoScan Pro that should do the following: read images from defined folder and re-save them without alpha to another directory (also user-defined) and then add re-saved images to the new chunk in the document:
import PhotoScan
import os, glob
from PySide2 import QtWidgets
path_in = PhotoScan.app.getExistingDirectory("Specify the path for input TIFFs:")
path_out = PhotoScan.app.getExistingDirectory("Specify the path for output TIFFs:")
app = QtWidgets.QApplication.instance()
print("script started...")
app.processEvents()
image_list = [photo for photo in glob.iglob(path_in + "\\*.*", recursive = False) if (os.path.isfile(photo) and os.path.splitext(photo)[1][1:].lower() in ["tif"])]
input_photos = list()
for file in image_list:
image = PhotoScan.Image()
image = image.open(file, datatype="F16", channels = "RGB ")
export_path = path_out + "//_" + os.path.basename(file)
image.save(export_path)
input_photos.append(export_path)
print("Written image ", export_path)
app.processEvents()
chunk = PhotoScan.app.document.addChunk()
chunk.label = "Re-saved TIFFs"
chunk.addPhotos(input_photos)
doc.chunk = chunk
print("script finished")
However, it may work quite slowly, if you have hundreds of images, so you can check it on the smaller sub-set (like 10-20 images).