There is an IP Pool of "Plesk Hosting" type named Pool.
An IP address from that pool is assigned to a Plesk client Client1 on the Plesk node Node1 but is not marked nor as a Lease neither as an Exclusion under
Top > Service Director > IP Manager > IP Pools > Pool. As a result, other Plesk clients can order the same address.
The address in question was assigned to Client1 manually, and that did not synchronize with OBAS because Node1 is not among Hardware Nodes which may use the IP pool as it can be verified under
Top > Service Director > IP Manager > IP Pools > Pool > Hardware nodes
Please add the address in question to the list of Pool's exclusions under
Top > Service Director > IP Manager > IP Pools > Pool > Exclusions tab > New Exclusion
To avoid the issue in future please make sure that IP addresses are assigned to nodes from corresponding IP pools.