I haven’t had this problem, but I’m assuming that the ANT+ frequencies and wifi frequencies are causing interference with each other as they are both in the 2.4GHz range. To test this further, try to see if the problem happens when casting with just your Zwift computer or other devices too. Also see if you can use a different device to record your power and HR (like a Garmin head unit). Perhaps if the Chromecast supports connecting to a 5GHz wifi channel, try that?
My “go to” recommendation for signal issues is to use a USB extender for your ANT+ stick. I typically put mine in the straps of my saddle bag, but I’ve also had success laying the ANT+ stick on the ground. right under the bottom bracket (although this is kind of far from the HR monitor). Just make sure it stays away from any moving parts.