Sharing in case anyone else has this issue: one of our teachers let me know that she was not seeing all of the terms when she went to Classes > View Other Classes and changed the year to All Year (for example, a yearlong course would show terms 1st, 2nd, and 3rd but not 4th). As context, we have not yet activated the new Portal Browse Groups feature.
I submitted a support ticket and they confirmed that this is a known issue which will be resolved when the new Portal Browse Group experience is activated in Core.
With the busyness of back-to-school we're trying to not overwhelm our teachers with changes and communications, so we're going to wait a bit before we activate it. It doesn't look like the "missing" term causes any issues (you can still see it in the gradebook), and it doesn't affect all of the courses for some reason, but it was a little alarming. Maybe Blackbaud will roll out a fix before December when the current Browse Groups view sunsets, but in the meantime just wanted to share to let others know!