Just ran into this issue myself, trying to Collect from a SQL Server using a Collection to Filter :
Collect ( Collection1 , Filter ( [dbo] , ID in Collection2.ID ))
Still getting the non-delegable flag and Powerapps telling me my 1-column (Collection2.ID) is not in fact a 1 column table ... And the documentation still says that "in" is delegable with SQL.
Yes unfortunately this has always been an issue, PowerApps doesn't delegate the 'in' operator to SQL. This has forced us to use a SQL Query from Flow for Azure SQL, and a Stored Proc from Flow for SQL on premises. Flow does 'n great job to retrieve the data, but it would be a glorious day when PowerApps finally delegates the 'in' operator.
Please try if it helps to use nvarchar type columns. In my tests the IN delegation is working , if the column in your SQL DB is nvarchar. On my case i added extra computed column to database just for this. Or then make a view if it suits your needs.
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