I had one of these very similar problem several years ago. It was a DX80 w/VM, but same problem. 2 of the lines disconnected the way they should, but 2 wouldn't disconnect reliably.
Went through all the possible scenarios, calling the line, getting put on hold, then hanging up from the distant end, system would either take forever to release or wouldn't release at all.
Called the CO to verify that CO disconnect was on the lines. They told me it was. Turned out that 2 of the lines were served by one switch and the other 2 were served by a different switch. Each switches timings were different. I could make the symptoms reverse lines by changing the timing in the DX80. Took some doing, but since I personally know the guys running the switch, I got them to put all the lines into the same switch to get the same timings, voila problem disappeared.
You may be up against a similar problem, and with an FXII, you can set each line different (50 or 300 ms) CO disconnect times. Might be worth a shot.
