This is a followup to the original post.
Paint.Net version 3.36.3158.38068
An image at 1000x8421 (8,421,000 pixels) saves.
An image at 2900x2903 (8,418,700 pixels) saves.
An image at 1000x8422 (8,422,000 pixels) will not save.
An image at 2900x2904 (8,421,600 pixels) will not save.
The trip point seems to be somewhere between 8,421,000 pixels (saves) & 8,421,600 pixels (will not save)
Notes:
Blank images will save.
Images with content will save.
Images will save whether 8 bit or 32 bit.
Hope this helps.
Rick