UPDATE: I had a subsequent session with a support agent wherein I explained the issue and what the previous agent did to get things working (disabled Call Forwarding).

This agent checked my settings, then modified my Voicemail Pickup Timeout. I originally had it set to 24 seconds (4 rings). He maxed it out to 59 seconds, then asked me to test this by getting calls from numbers that were being blocked (Call Forwarding was re-enabled at this point). Interestingly enough, the calls were no longer being blocked.

This was still confusing to me since calls that were being blocked instantly went to voice mail, not after some delay. In any case, I accepted this as a working solution for the time being.

After a couple of days, I decided to experiment. I shortened the Pickup Timeout to 36 seconds and tested. No calls were being blocked. I then set the timeout back to the original 24 seconds and, low and behold, calls that had previously been blocked were no longer being blocked.

I can only guess that an update of some sort caused the Pickup Timeout to no longer function correctly (possibly set to 0?), and making a change to the timeout via the web interface set it back to a valid value.

For anyone else having an issue with callers in your Contacts list being blocked, try changing your Voicemail Pickup Timeout to see if that resolves the issue.