grid spacing incorrectly saved when changing Document Scale

Summary:

When saving the document, the grid spacing may be incorrect (reopening the document gives a different grid). This may be related to changing the scale.

Steps to reproduce:

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

What happened?

The grid has changed.

What should have happened?

The grid should have remained the same.

Note also that if steps 4 (Spacing X & Y) and 5 (Scale x) are swapped, then the saved SVG file is different:

--- drawing1.svg        2020-12-18 17:45:49.152929362 +0000
+++ drawing2.svg        2020-12-18 17:47:24.032078942 +0000
@@ -38,8 +38,8 @@
     <inkscape:grid
        type="xygrid"
        id="grid10"
-       spacingx="2.6458333"
-       spacingy="2.6458333" />
+       spacingx="5.2916667"
+       spacingy="5.2916667" />
   </sodipodi:namedview>
   <metadata
      id="metadata5">

Version Info:

- Inkscape Version: Inkscape 1.0.1 (3bc2e813f5, 2020-09-07) / Debian package inkscape 1.0.1-2
- Operating System: Linux (Debian)
- Operating System version: bullseye/sid