Have you implemented Row Level Security for your dataset?
I have reproduced this issue when I set Row Level Security for my dataset and don't add the members in the group to any of roles, then read-only members will not be able to see the data. Editable members can still see the data because Row Level Security roles only apply to read-only members. See more details here.
If that is the case, please make sure you have added all read-only members into the corresponding roles, then they should be able to see the data.