Hi Gourav,
This is the correct standard behavior. The same is mentioned in other note 1638210. If you want to use it as multi mixed then put "can be used by several operations" but in realty I assume you are not using it in several operations (so no negative impact). Then try to CIF the same to APO (the changes, if doesn't change then you need to delete PDS/PPM being used, thereafter resources and finally CIF them again). First try the same in quality environment and see impact.
1638210 - Resources created in APO as category "multimixed"
Or check for a work around with below link and mentioned SAP note in the same. If you have above situation then create directly in APO from single/multi activity resource which is CIFed from ECC. Though this way you will have two but will meet your requirement (keep in mind how to incorporate changes in both resources while testing)
I hope it resolves your issue.
Regards
Rahul