For some years now the action of installing a later version of the software automatically removes the earlier one. The only exception to this appearsΒ to be if your later version is a different edition to the earlier one, e.g., you previously had Premium and now have Suite.
Let me guess, you have trouble with SD analogue video in MMS2023. π
Having both installed can be done. Magix gave me a download link so that MEP 2021 will stay installed. I suggest you contact Support, tell what you want to do and why, and see if they'll give you a link to MEP 2021. As Jeff says, it's an issue with the particular editions interacting.
Click SUPPORT at the top of this page, then click the "Movie Studio" button, then go down to the bottom, on the left, click 'Contact Support". You'll then get a link to the Support email. π