Conceptually when you're casting your S21 to your Sony TV it's not a direct connection between your two devices. Your phone and your TV are both connected to your router and it's within your home local network that all the interactions take place. So if casting has stopped working recently and if both devices are connected to your home network without any issues, than yeah perhaps some update is the source of a problem. But there's no definitive proof of that. Instead of blaming the update, you need to determine it's not just some other issue that's the actual source of this problem.
-- Your S21 is presumably connected to your home network using WiFi, yes?
-- Do you have your Sony TV connected to your home network via WiFi or an Ethernet cable?
-- Is your phone having any problems with other online connectivity issues or is it just the casting problem?
-- Same question with your TV?
-- Log into your router's setup menu and confirm both your phone and your TV are connected.
-- If your TV is using an Ethernet cable, are you sure it is still plugged into both ends,and that check that it hasn't been damaged.
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