strange problem with focus 14 blue braille display paired with an iPhone

I have had this problem for some time and have tried with several braille displays in the same model, which means this is not specific to my own display. When I write, especially in long texts, the display randomly disconnects and VoiceOver does that bleep sound and then it reconnects with another bleep. Often the last things I type go away and I have to retype them. The display is brand new and I have tried with several and just don't know what is wrong.