The default mech is EXTERNAL and in ipa-adtrust-install the root user is mapped to the directory manager which does have permissions to modify cn=config.
However, if the user has configured SASL_MECH configuration option, e.g. to 'SASL_MECH GSSAPI', we will bind as admin user, which does not have the permissions to modify cn=config.
This can be reproduced creating a /root/.ldaprc file with the following content
Duplicate to #3895.
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