First capture on my Deardorff 8X10. Kodak Portra 400. Sunny 16 day with hazy background. Wollensak 159 at f/32, shutter 100th sec. Any guesses as to what caused the "glare" artifacts bottom center and bottom right?
Check the back for debris built up in the corners preventing the sheet film holder from sitting flat in the back.
Darkslide slot of the film holder leaking light.
Bellows problems as mentioned previously.
And another suggestion: do your camera/process testing with B&W film. Costs about 1/4 as much per sheet, and less than half as much to process (and more accessible to develop yourself, which is far cheaper yet).
This is reversed for 35 mm -- B&W costs at least as much to process, and consumer color films (Fuji Superia, Kodak Pro 100 and Gold 200) are actually cheaper than B&W.
Yes, check your bellows, back and lensboard for holes/leaks. However, most of the light strikes like this that I see are caused by inadvertently pulling the camera spring back open when inserting or removing the dark slide. Pinch the spring back to the camera body with your free hand when removing and inserting the dark slide.