It occurs to me that the gamma ray hypothesis has a fairly easy check. Light sources pass through the telescope’s optics (typically mirrors or occasionally lenses) which leads to a characteristic “point spread function” for point sources like stars. If it were an errant gamma ray exposure directly on the film, it’s extremely unlikely to have the PSF of the standard light sources.
You can compute the PSF from known stars on the same image and run a statistical test, but TBH just visually comparing the transient with a few stars of similar brightness on the same image should put this one to rest.
Following up on this, I eyeballed the images in this one: https://iopscience.iop.org/article/10.1088/1538-3873/ae0afe
The brightest stars in all of the images have a clear 4-pointed pattern. The brightest transients _do not_ show this pattern.
This is obviously not definitive, and the fainter stars are harder to eyeball the PSF, but it does provide some evidence to support the hypothesis that the brighter transients could be due to gamma ray exposure of the film rather than flashes in the atmosphere or space.