Janice,
I just ran a test and replied via my email client--it posted as a
separate topic and then replied again directly from the group and that
one ended up within the original Topic.
Most likely that means that your email client isn't supplying either the In-Reply-To or References, or else Groups.io is having some trouble parsing those fields.
I use Outlook on a Windows 10 computer.
I don't use or know much about Outlook, but if you reply to this message the same way that you normally do in your other groups (that is, using Outlook rather than the Groups.io pages) I'll take a look at the View Source of your message and see if I can tell what's happening.
Shal
Shal Farley