You've done the standard troubleshooting steps for what you've reported. I will add that the Roku 3 is getting some age on it. The model 4200 is from 2013, and the model 4230 is from 2015, so issues beginning to show in a device that old isn't that unusual. I had a Roku 2 model 4210, which is identical to those models, finally go out a couple of weeks ago. I still have a Roku 3 in operation. They've been great devices, and while I'm not suggesting you replace it, keep in mind that it's getting on in years, and the day will come, sooner than you want.
As for the Roku Ultra, they were introduced in 2016 (model 4640) and updated in 2017 (4660), 2018 (4661, but it's identical to the 4660), 2019 (4670), and 2020 (4800, current model). So, if it's one of the older models, it's not that much more recent than the Roku 3, and could be getting on in years. Still, I would expect the app to work properly.
I wish I had some suggestions for you, but I don't. I just wanted you to have the background and timeline on those devices, if you didn't already have that, so you'd be well informed if it comes time to make a decision.
If you were to replace one, all the current Roku lineup, except the Streambars, have the same processor. RAM is the main difference between the core line up of devices. If you wanted to pick up a replacement for the Roku 3 locally, if it didn't make a difference, you should be able to return it. Check the retailer first. But if it makes a difference, that pretty much cinches that Amazon has a recent update that adversely impacts older supported devices.
Roku Community Streaming Expert
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