A purpose is set by default in libssl client and server certificate verification routines, but it can be overridden or removed by an application. Therefore, where a purpose is set the certificate chain will still be rejected even when the strict flag has been used. All of the named 'purpose' values implemented in libcrypto perform this check. If a 'purpose' has been configured then there is a subsequent opportunity for checks that the certificate is a valid CA. This effectively bypasses the check that non-CA certificates must not be able to issue other certificates.
An error in the implementation of this check meant that the result of a previous check to confirm that certificates in the chain are valid CA certificates was overwritten. Starting from OpenSSL version 1.1.1h a check to disallow certificates in the chain that have explicitly encoded elliptic curve parameters was added as an additional strict check.
The X509_V_FLAG_X509_STRICT flag enables additional security checks of the certificates present in a certificate chain.