Hi @AWagenhurst ,
When deploy application or update to different forest, configMgr doesn't care about the domain membership of managed systems so multiple forests, domains, and trusts between them are irrelevant.
It will not install the APPS, please check execmgr.log and Smsts.log to see if there is any error.
There are three articles detailing the deployment of SCCM in different forests. We could refer to the following.
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