We have investigated this here and have unfortunately detected an issue in a combination Berlin / Android 6.0. This issue is also reproducible with TMemo and has already been reported to Embarcadero. The issue should be solved by Embarcadero, but a possible workaround is to install a swift keyboard as mentioned in the following QC report:
It seems to be an issue with the default Google keyboard in Android. A workaround is to install the Swift keyboard and use this instead, which will properly handle text input in TMemo and TTMSFMXRichEditor
With newer Delphi releases the problem got worse unfortunately. Embarcadero removed keyboard handling methods from the FMX base class interfaces and these methods were essential for the keyboard handling we need in Android. In short, it is not longer possible to handle individual keystrokes from the control. Workarounds for this are very complex and although we spent already hours and hours on research, we haven't found an adequate replacement for the removed interfaces by Embarcadero yet.