Some OOTB attribute lookups seem to have a lot of records shown when filtering. For example, in Project Grid when filtering on Manager (Project) all the inactive resources in the system are shown.
Resolution
Any filters that are based on OOTB lookup attributes will be the same as in Classic.
Manager on Project is based on Browse Resource lookup.
If you edit this lookup then this will also affect Classic.
You can use access rights to restrict access, you can provide access rights to resources by OBS or Instance rights, this way they won't be seeing all the users in the system in the lookups.