Normal homes these days have quite a bit of competition for wifi. Hell even today's watches these days have wifi. Add a kindle, a few security cameras, a few laptops/tablets, a few phones, etc. Then add some crappy/noisy LED bulbs, a cheap microwave, and various consumer electronics and wifi is far from a sure thing for streaming 4k video over... especially for hours at a time.
I had some network lag/dropped frames, turns out my stereo receiver was downloading a quite large firmware update. Had me freaked out a bit, I hadn't recorded the mac address. Didn't figure it out till I turned the TV on and saw a dialog asking if it should upgrade.
So, yes, generally I'd recommend that anything that's going to consume hours of 4k video be put on ethernet if at all possible.
I always try to connect devices that don't move to Ethernet. It saves WiFi bandwidth for "mobile"devices and is extremely stable and the full bandwidth is always available.
Things like SmartTV, RPi, Solar Panels, Amplifier, etc. I hate it that Chromecast only has a wireless option.
But more on-topic, it shouldn't drop off WiFi just like that.
From what I've seen about Smart TV's, I'm better off not allowing them to ever connect to the Internet. Forgo all the "smart" features and just use a Fire TV stick, Roku, Chromecast, etc.
My current TV is an 8 year old 46-inch "dumb" TV, but I plan on upgrading to something bigger later this year, and it looks like my only options will have Smart features, but I already use a Fire TV stick and a RPi, so I don't think I need the smart features.
It's 2020 now and my experience is that WiFi is wonderful, yet still too unreliable to count on 100%. At some point you're gonna run into issues if you run something that's meant to be 24/7. What's so hard about cabling?
If you're living in an apartment (like a sizeable chunk of the world's population does), WiFi is absolutely not an option for anything even remotely traffic heavy.
I had some network lag/dropped frames, turns out my stereo receiver was downloading a quite large firmware update. Had me freaked out a bit, I hadn't recorded the mac address. Didn't figure it out till I turned the TV on and saw a dialog asking if it should upgrade.
So, yes, generally I'd recommend that anything that's going to consume hours of 4k video be put on ethernet if at all possible.