I worked on a document where the grid spacing was 10, but when the
document was saved (and opened again), the spacing changed to 100.

In the diff of the SVG file after reopening the document, fixing the
issue, and saving again:

@@ -43,8 +43,8 @@
        type="xygrid"
        id="grid833"
        units="px"
-       spacingx="100"
-       spacingy="100"
+       spacingx="10"
+       spacingy="10"
        empspacing="1" />
   </sodipodi:namedview>
   <metadata

Hi Vincent!

That does sound like a bug.
Could I ask you if you could file this directly upstream at
https://gitlab.com/inkscape/inbox/-/issues/ ?  Then you could verify
this directly with them, without me acting as the middleman.
If you do, please report back here a link to the issue so that it can be
cross-tracked.

Hi Mattia,

Done, with a reproducible testcase.

Thank you!

Retitled according to comments

https://gitlab.com/inkscape/inbox/-/issues/4205#note_470326556

and

https://gitlab.com/inkscape/inbox/-/issues/4205#note_1224138272
  "The grid now updates when scale is changed, in line with
  #4205 (comment 470326556)"

The testcase for 1.2.2-2:

1. Run inkscape
2. Go to File → Document Properties
3. Go to Grids, then click on New
4. Set Spacing X & Y to 10
5. Go to Display, then set Scale to 2 (default was 1)
6. Close the "Document Properties" window
7. Save and quit
8. Run "inkscape drawing.svg" to reopen the document

The grid has not changed at steps 5 and 6, but it has changed only
at step 8.

Thanks for the update, including the fact that this has been fixed,
although I'm a bit sad that this fix seems to be accidental…

FWIW, It's not clear to me if we will be able to include inkscape 1.3 in
bookworm, as the timing is a bit tight IMHO.