Acrobat Reader DC - multiple tabs

Hello.

My Acrobat Reader DC version is 2019.021.20061 (File version : 19.021.20061.33636)

I used 'multiple tabs' for reading docs by Acrobat Reader DC.

(At that time, 'multiple tabs' function was operated well)

However, for the last few days, 'multiple tabs' has not been operated. When I opened the new pdf files, the new window was opened. 

I tried to change the preference.  (Edit>Preferences> General > Open documents as new tabs in the same window(requires restart) : checked))

After restarting Acrobat Reader DC, the preference is changed back again. ( Checked -> Unchecked )

I even tried to restart my PC, too. 

Also, I deleted Acrobat Reader DC, and installed again. Nothing works.

I clearly remember that multiple tabs work well, I don't know why it does not work now.

Please help me to use 'multiple tabs'. 

Is there another way to change the option (preferences) for that function ?

Regards,