In fact, the Government of Pakistan, Ministry of Maritime Affairs (Ports & Shipping Wings) published a circular in which clarified its stance on scrubbers:
The discharge of wash-water from open-loop scrubbers is prohibited in Port. While in the Port, vessels fitted with hybrid type of scrubbers should switch to the closed-loop mode of operations. As for vessels fitted with open-loop scrubbers, they would need to switch over to compliant fuel instead.
Meaning that the circular recommends vessels to switch off the open-loop scrubbers before their arrival in ports waters. In that case, it will be easier to prevent and deal any operational issues on time, before entering Pakistani waters.
Moreover, Pakistan's Ministry of Maritime affairs advises members with ships fitted with scrubbers, to take a note of the above-mentioned circular and follow the local regulations.