So I did some further testing, and it appears the problem is not just with "trunk" ports between the two switches. Even tagged VLANs between ports on the same switch are having the problem. See the...

I have a problem where JetStream switches (T1700X-16TS and T1700G-28TQ) are not properly passing traffic when using ports with tagged 802.1Q VLANs. When using untagged VLANs, there are no problems....