While enabling Magisk Hide on the usual suspects, I found myself wondering if Magisk Hide would prevent a malicious app from detecting that the device is rooted, thereby preventing that app from taking full advantage of having root access. After all, if non-malicious apps that check for root access can be "fooled" by Magisk Hide, wouldn't that happen to malicious apps as well?
Taken from the Magisk site here:
Magisk Hide is the feature which is available in the Magisk Manager application and if you want to hide the root permission for the particular banking application.
You can turn the Magisk hide option from the settings of the magisk application then you can easily enjoy your favorite app without any issues.
This means that you have to turn this feature on, then select the app from your list of installed applications that you want Magisk Hide to work for. In the example on the site, they list banking apps, Pokemon Go, etc. To answer your question, Magisk Hide would not prevent a malicious app installed on your system from "seeing" or accessing root unless you enabled it on that particular malicious app specifically.