So I've noticed this bug that may or may not be affecting most readers. So last week I got a new iPhone 6S. When I was done setting it up. It just so happened to already have iOS 13.3 preinstalled. Now the bug. I went to go download Samantha (Enhanced). The voice only lasted max 30 seconds before reverting back to Samantha compact.
My iPad Pro (2017) was already running iPadOS 13.3. Since I didn't install Samantha enhanced on that device and it worked fine over the course of 1 week. The storage is barely used. I have a 32GB model, and only 10GB of it is filled, so I don't see why it won't work for longer.
My brother has a 6s as well, iOS 13.3 and his phone uses Samantha enhanced like normal. Something I forgot to mention is that all the other voices downloaded successfully with no issue.
I have reported this bug to Apple Accessibility, but so far I haven't gotten a response. I will report back if I get one.