After upgrading IDM from 12.6 to 14.3, post-upgrade, blank spaces are allowed in the 14.3 version. The password policies are same in the both versions.
Password policies have always allowed for blank spaces in the passwords. There has been no change to the functionality from one version of IDM to the next regarding spaces in the passwords.
Check for custom code in your previous version that prevented spaces in the passwords.
To prevent spaces in passwords consider creating a Regex (regular expression) in the password policy.