Eric, I am an avid Zwift user and have noticed some bugs as well. Zwift has a pretty good troubleshooting forum
https://support.zwift.com/hc/en-us/community/topics/200165969-Tips-and-Troubleshooting?page=2#posts
First, I have a PT hub and a dumb trainer so my power numbers are coming right off my power meter, and are as accurate as my power meter. I would order the dongle for your laptop, its pretty cheap, but if your trainer is not accurate, then you can go back to whatever is reading off you computer; and get what you think are the most accurate readings. What is the Bryton reading off as far as power readings? If its just making estimations based upon speed, then it might not be as accurate as the Tacx reading. So I would zero out your powermeter if that's what you are using, or recalibrate the bryton.
Second, the Satori is just taking your speed, cadence, and the known resistance parameters of the trainer and calculating watts based upon that algorithm. Zwift is purporting to do the same, and includes data concerning the alleged elevations of the course. Again, because you are not pulling real power numbers, and they are estimates, it may just be that you need to check your settings. Zwift may think you are using a dumb trainer and making additional adjustments, but Tacx might be making the adjustments based upon the resistance. So, it may be when the new software was downloaded, it mistakenly reads your trainer as a dumb trainer and is doing the algorithm for a dumb on top of the Tacx algorithm, which is making resistance adjustments on the trainer.
Third, as mentioned, Zwift has lots of bugs, my latest one is that it sometimes kicks me out of a group ride when the course hits a fork, which is lots of fun. I would post an inquiry on that troubleshooting forum.
Fourth, Tacx makes great products but has a terrible quality control program sometimes. You can get a bad run on a product which otherwise has a good reputation. I would have a buddy with a good power meter take a ride on your trainer and see where the problem is coming from, is it Tacx, or Zwift
