Group By feature does not show full list of items

This issue just started very recently.

We have a SP list we use to track attendance, it currently has about 4000 rows of data and one of the columns, Agents, has about 100 different names.

When I try to Group By the agent column, instead of getting a grouped list of all 100 agents, I'm getting like 5

We have changed nothing on our end.  Anyone else experiencing this issue?