It’s not uncommon to register a plugin on RetrieveMultiple message and fiddle with the output, e.g. drop some values, add some calculations, etc.
Chances are that you are changing the QueryExpression, and the plugin is working fine in the classic UI but you will have challenges migrating to Unified Interface.
The reason is that in the Unified Interface Dynamics 365 uses FetchExpression, so you’ll need to handle both cases if you want to use both classic UI and Unified Interface.
To reuse as much of the existing code as possible, use FetchExpressionToQueryExpressionRequest to convert, then do what you need to the QueryExpression, and then change it back with QueryExpressiontoFetchExpressionRequest. Sample code is available.
Cover photo by Lucas Vasques