For Icecast server accounts there are no crossfade settings - please see the screenshot.
Those settings present if I log in as admin and log in as a client, but for some reason they not present if I log in directly as the user.

What is the reason for this and how this should be fixed?

This is the intended behavior. Those settings force re-encoding to be turned on in ices (which in turn increases CPU usage) and as such are only available to the administrator.
If you would like your clients to be able to make the adjustments themselves then please refer to the CLIENT_ENCODER_SETTINGS in /usr/local/centovacast/etc/centovacast.conf