I had the same problem and this probem started in R2020a.
Every time you started MATLAB R2020a, such a temporary folder is automatically created and added to the MATLAB path but it was not saved. When MATLAB exists, the temp folder is automatically removed. The name of the folder changes every time. You can check it.
So if you install a third-party toolbox and during that process, savepath() was executed, then this temp folder is saved thus caused the warning the next time MATLAB is started.
The robust solution is to remove that temp folder before savepath() is executed in the installation script.
If the installation script can't be changed, the manual process is to remove those temp folders and save the MATLAB path once for all.
The problem has been reported to the Mathworks. Didn't follow up on bug report since it is not that serve.