My guess is that if you switch the existing Design Modeler License to Discovery, Design Modeler does not seem to work as agppi is deleted from the list of licenses.
Perhaps in the version after 2021 R1, the license required for Design Modeler is changed to be driven by disco_level1, so it is assumed that it works normally. However, in the previous version, agppi is requested to the License Server, but the Server replies that there is no such license, so Design Modeler operation seems to be forcibly suppressed.

Again, Design Modeler works fine in 2021R1 and later versions. Check out the following:
1. Make sure your License Manager is 2023R1.
2. Check if the license is Design Modeler or Discovery.
(Check the License File to see if agppi is present and disco_level1 is present. Probably not at the same time. If it is agppi, you are right. But my License File issued in April 2023 has no agppi and disco_level1 is 3 There are dogs.)
3. Check if the License File was issued this year.