I formatted a column of numbers in the menu Format - Cells - Numbers as

##_i##_i##_m##_i##_i##

i. e. I want to have small spaces ("_i")and a big space ("_m") like written in
the help. The issue is the same if the format is more simple.

After closing the dialogue box the formatting works as expected. But after
closing  the document and oppening it again the format has become

#"       "

i. e. the spaces in between the digits have vanished, but there is a string of 7
spaces at the end of the cell, which shows also in rendering.

If you use spaces in quotation marks like

##" "##" "##"  "##" "##" "##

instead, the format is retained also after closing the document.

At least the documentation should be changed accordingly so you can insert
spaces that survive the closing of the document even if you can't determine the
width of the spaces in the workaround.

Jens