Response from HTC (UK):
"I can't see anything in the publicly available details that tells me it is no longer working but it does sound like a compatibility issue. Have you reported the issue to Sony, they may be aware of the cause. "
When I pointed out that screen-mirroring worked before the Sense 6 OTA update, and so the problem was likely introduced by this update, I got the following response:
"At this point the only make that we have had confirmed with so far is Sony which is why i suggested contacting them, we are investigating the issue here but have as of yet been unable to find a cause. It is very possible that the issue occurs with other makes but we have as of yet not been informed of any beyond Sony. If it is caused by something specifically changed with the Sense 6.0 update that isn't governed by changes in the Miracast certification system then we will certainly address it."