Hi @Anonymous,
Could you please share a screenshot of the configuration of your flow?
How many groups does you have in chatter?
Which action of chatter connector that you mentioned to only looks up a limited number of groups?And waht is the limit number?
I have made a test on my side to create a group in chatter,and I could see the group in the connector.

Please share more details so we would try to provide a proper workaround for you.
Best regards,
Alice