aliasclients has nothing to do with Pi-hole's group management clients.
It's a somewhat experimental feature that would allow you to aggregate several client IPs under the same aliasclient name in Pi-hole's dashboard exclusively, see e.g. Aliasclient usage and queries - #5 by Bucking_Horn. It has no UI configuration component, other than enabling related debug information output (via debug.aliasclients).
Did you actively use that feature in v5, by manually inserting entries into the aliasclient table via SQL statements?