Per Microsoft documentation, No, there shouldn’t be a scenario that a security content contained within a monthly rollup is not available anywhere else. Microsoft each month will release “security only” and “monthly rollups”. The monthly roll up includes security only as well as fixes from all previous monthly rollups.
You may want to read through the Microsoft blog for understanding Microsoft policy on security only and monthly rollups.
Below are few selected snippets from the Microsoft blog post:
“A security only quality update
A single update containing all new security fixes for that month”
"A security monthly quality rollup
A single update containing all new security fixes for that month
(the same ones included in the security only update released at the same
time), as well as fixes from all previous monthly rollups. This can
also be called the “monthly rollup.”"
"For organizations that typically deploy only security fixes, you will
now find that instead of approving or deploying a set of fixes each
Update Tuesday, you will approve or deploy just a single update.
Since the security only update and the monthly rollup both are
published using the “Security Updates” classification, existing
automatic approval rules in WSUS would approve both the security only
and the monthly rollup each month. The same is also true with
Configuration Manager automatic deployment rules. This will require
either manually approving or deploying updates each month, or in the
case of Configuration Manager, adjusting existing automatic deployment
rules. See the previous section for details.
You install all security updates as we release them, and some non-security fixes to address specific problems
Since the organization will typically be deploying only the security
only fix, see the previous section for full details. In cases where
there is a need to deploy one or more non-security fixes, manually
approve the latest monthly rollup that contains the needed fixes. This
monthly rollup will contain other fixes as well, so the entire package
must be installed."