Just a disclaimer: I have an iPhone 5S and my Mac is mid 2011, so some of this could be due to how old my junk is, but just trying to help others who have experienced this issue. Read on!
I find sometimes my signal drops mid ride. Sometimes it comes back --easy-breazy, right? Other times, when the signal comes back (usually after a long delay) it will set me at a particular watt output and stay there despite my effort.
The past couple times this has happened, I’ve found that I’ve had some luck with the following:
Turn bluetooth on the phone completely off and shut down my Zwift Mobile Link (keep the computer app open).
Restart the ZML, then the phone’s bluetooth receiver.
Go to computer Zwift app > Menu > Pairing. Take off the device that is paired (smart trainer), and then re-select it.
You might have to try step 3 a couple times, but it’ll fall back in line. Hope this helps! Good luck with your riding! (And troubleshooting!)