First of all you should know that in your cases, you are using 2 different structures let check case 1:

The discussed problems are not serious. How

do I ask for explanation of “The problems discussed are not serious” (Example: “Is there an argument in favor of this approach? (B)

in (A) you are using adjective+noun whereas in (B) you are using adjective clause.

When you are using adjective clause, you have the power to mention the time, i mean in (B) you can say: The problems that are discussed (are going to be discussed) are not serious. If there are any problems that haven’t been discussed yet, why wil i be taken out?

For any one who is interested, these are not serious problems. It means they discussed the problems. So it means they discussed the problems.

In A, you say the discussed problems in PowerPoint. the discussion has ended.

In a situation where the discussion hasn’t started yet isn’t serious, but it is true, saying the discussed problems are not serious.