I too have not been able to mirror to a Panasonic TC-65AX800U using Miracast with my SP3. I can, however, mirror to this TV using my Surface 2 and an Asus Laptop each running Win 8.1.
I have reset my SP3 to out of box condition and tried to mirror before applying any updates. No dice. I fully updated and still no dice. Clearly the Panasonic supports Miracast as evidenced by two of my three devices connecting.
MS, please acknowledge that this is a known issue the SP3 or I am going to return it.
I can't find any reference in the user guide http://shop.panasonic.com/shop/model/TC-65AX800U?t=manuals&support#tabs or on the web site tat this TV is Miracast certified. Do you have a link? There's no guarantee that any device will work with a non certified device.
How is this constructive?!
The user clearly stated that the mirroring function works on SP2, so obviously the problem lies with SP3 which btw was marketed as a product which was able to use screen mirroring with smart tv:s.
This "blame game" between manufactures is best kept on a B2B level. As a consumer I do not care about which company failed to e.g. implement proper miracast protocols. Just get it done MS or I'll too will return this otherwise fine product.