As hinted here I can confirm that messages containing spaces in the "address" field are not imported by the current version 2.4.0.
For me this was quite a big issue, as lots of messages had their addresses formatted like "0123 4567890".

I'm submitting it as a separate issue so people can find it easier via the search function.

Here's the workaround that worked for me:

• Extract messages.ndjson for the .zip file created by the backup function
• Search and replace with a matching regex. I used search for ([0-9]{3,6}) ([0-9]{6,10}) and replace by \1\2 in xed.
• Save the file and zip it again together with the "data" folder from the original zip.
• Import this newly created zip file

Would love to see this fixed in an upcoming version.