Findstr is a simple 'show the text' program with no way to extract part of a line.
Findstr is also buggy in many ways - there is a good page by dbenham on all the issues.
You can use a for /F
command to extract parts of the line. Knowing the exact text in the line up until the part that you want is important - if there is an equals sign between the two parts (and no other equals sign before that) then it's straightforward.
for /f "tokens=1,* delims==" %%a in ('findstr "ro.redacted.version.release=" "c:\folder\file.txt" ') do echo "%%b"