Since 3.9.18 firmware for 6600M radio freeze/reboot/lock-out doing 2BSIQ

Ever since updating to 3.9.18, I have been having a recurring problem, as described below. Note that before this, I was running not on the previous firmware version, but the one before that (sorry, I can't see the firmware versions for the reason you're about to hear about).

I am operating a 6600M remotely, using a Maestro and SmartLink, doing 2BSIQ. I am running N1MM remotely (not at radio site) and generating CW from N1MM. I am using a RemoteRig for audio and manual keying (though I usually don't use the paddle.) I have been doing this for several years. In the last 6 months or so, it was very solid. But ever since the 3.9.18 upgrade, it has been basically unusable. Here's what happens:
I operate with two keyboards and CQ on alternating bands, interleaving QSOs. I am fairly careful to hit ESC before switching, if the other TX is not done. (I can't 100% swear that I don't sometimes type on the other keyboard while TX is still in progress, but this used to interrupt fine and work properly.)

I start the CWT and within a few minutes, the following happens:
The transmission stops and N1MM gets stuck thinking it's sending a message (box remains bordered by red). The Maestro screen goes blank (panadapters disappear). A few seconds later, "Select Radio" comes up and it shows me as still connected and the radio as "in Use". Precisely, it says "In Use (multiFLEX disabled WD6T)". (A similar message appears if I don't disable multiFLEX). If I then try to connect to the radio, I am unable to do so. I can select "Disconnect …" and then it shows the radio. I click on it, and then "Disconnect Station". Nothing happens. I am stuck in this state until I reboot the radio.
I also tried logging into a computer at the radio site and launching SSDR from there. It too is not able to disconnect the radio. So it is clearly the radio that is in a bad state.

Again, this is a NEW problem and I'm about 80% sure that it's a bug in the latest firmware because nothing else has changed.

I've heard it is not advisable to back-rev the firmware, so I can't try that.
If you have experienced a similar problem, please let me know.

Flex, if you are listening, I would be happy to provide more diagnostics if you can tell me how to do so. Perhaps there is some log on the radio itself that could provide a clue?
I can be reached via the email on QRZ.COM

73
Dave
WD6T