Quote:
...snip....In Preferences\Default Grid, and under Spacing, I have entered a grid size of .100—the size I've used for months.
Your problem is using real world units (inches / millimetres..etc) while Gimp used pixels.
This is the default Gimp 2.10, where the default image uses a resolution of 300 pixels-per-inch (ppi)
The default grid settings is 10 pixels (very small spacing IMHO) and that gives a inches value of 0.033 inches.
Attachment:
01-default.jpg [ 95.36 KiB | Viewed 135 times ]
If the image ppi changes, and that can be the default value you set or the ppi of the open image the spacing in inches will change. The spacing in pixels still the same-old-10 pix. This if you changed the default ppi to say 96 - a common value for some images - the grid spacing is now 0.104 inches and still 10 pixels.
Attachment:
02-ppi.jpg [ 98.6 KiB | Viewed 135 times ]
The bottom line is, there is no automatic setting for inches spacing, only for pixels spacing. For inches, yes you will have to change this on an image-by-image basis and no it is not a bug.