Some emails appear without spaces between the words. Other emails appear normally.
View an email coming from, say, Quora. You’ll see that the spaces between the words are missing.
I expected all emails to show up properly.
Are there particular sources whose emails are always wrong, or is it more random?
I know other people using Arch-based distros have reported issues here. FWIW, I know some of the builds available (even the official builds!) seem not to be without problems. I’ve found the snap on Ubuntu-based distros to be less reliable than the deb, for example. Which is to say: it might be worth trying a different build and see whether that resolves the issue for you.
Is the raw message OK, or are there any other signs in the raw message that might indicate why there’s an issue? (I do seem to recall someone else reporting missing spaces in emails on this forum in the past few months so it might be worth tracking that down to see if they found an answer.)
The source code of the affected emails looks fine to me. I can’t see why this is happening. I can send you the source code of one of the emails, if you like.
Wouldn’t be much help to me, I’m afraid - I’m just another user. 
It happens with me too. Also using Arch with KDE, and Mailspring 1.9.2 installed with AUR.
Inspecting the Quora messages with builtin devtools, I see that happens when the font is “system-ui”, which computes to the default GTK font, Cantarell.
If I set any other font, the issue goes away. If I set “Cantarell” to any element, the issue shows up.
Also, this happens with Mailspring menus for me, maybe it is related.