FWIW, I tried sending an ePub using Kindle for Android send to Kindle feature. Sending worked but conversion failed for some reason. It opened just fine with an ePub app on the device.
I use the iOS app routinely to send PDFs that open in browser. That stopped working with Safari some months ago, but it works with Edge.
So yes, it's more predictable to use the web page than the apps.
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