Most of the time it's working with nav buttons but I've been a victim like you've been.
When it went wrong I'd refresh or navigate forward back & hope for the text to reappear.
I haven't seen it cut my text more than a handful of times so I've accepted it happens but as a safety I'm getting in the habit of avoiding it.
I have been a victim of this several times and I had no option than to start afresh to type again. I was so pained that instead of me to raise a topic here, I rather had to write a review to my mobile phone producers that they should make a function that will only allow back button in their mobile phone to work when you double click. Maybe you can set it to work once, but for the sake of BTT, I would set mine to work when I double click. My this, I will not mistakenly click the back button.
I don't know if BTT save cookies for security reasons, since the platform has been hacked before.