Symbols are scaled to px when inserting in new document

Edit for visibility by @jhofinger: unit, units

Steps to reproduce:

Step 1 - create a new symbol set:

• Open a new document
• Create an square with a side length of 30 mm
• Open the symbol dialog
• Add the element as a symbol
• Save the file as a symbol library

Step 2: insert symbol in new document:

• create a new document
• insert symbol from the symbol dialog

What happened?

The square will have a side length of 30 px, not 30 mm.

What should have happened?

The square should have a side length of 30 mm, not 30 px.

Inkscape Version and Operating System:

• 1.0rc1 (09960d6f, 2020-04-09)

• Inkscape 0.92.5 (0.92.5+68)

• Operating System: Linux

• Ubuntu 18.04.4 LTS

I believe this issue has already been reported here and has been set to solved, but for whatever reason it seems to be reappearing with me.