For example, following commands will return the same set:

openssl ciphers DEFAULT
openssl ciphers DEFAULTz

despite DEFAULTz not being a valid cipher string.

I have already reported this to Fedora. According to @t8m, the DEFAULTz is equivalent to DEFAULT:z, in which case it would be valid behavior (because z, not being a valid cipher string can be ignored). ciphers(1) does not seem to allow such interpretation, though (or at very least this is very counterintuitive).

I'm observing this with openssl-1.1.0g-1.fc26 on Fedora 26.