Unfortunately, Plesk doesn't support this behavior, because creating reverse zones for subnets smaller than 255 addresses is a problematic issue that can be solved in compliance with RFC2317.
Network /32 is merely a special case.
That is, the zone /32 also requires the creation of reverse zone according to RFC2317, because when you create a zone in the usual case, you have to create the zone:
123.123.123.123.in-addr.arpa
but there is still need to add a PTR record descriptions to describe the reverse resolving:
123 IN PTR test.plesk.com.
But it must match the address 123.123.123.123.x (123.123.123.123.123) that is no longer IPv4 address.