I’m just starting with Defold and was hitting my head with this wall, i am using Linux MX and have the same problem, using space only shots diagonal “TOP-UP”, i didn’t meet this issue before and the not logical behavior of the problem cannot allow me to sleep.
This is not related with Defold, the problem is reproducible here https://keyboardchecker.com/
And the reason for this already is documented here Key rollover - Wikipedia.
Using Z solve the issue for me too, thanks.