V24 (v23 is similar) and Win10/64. See below. FINDSTR works in a pipe. After switching to a new console screen buffer and returning, FINDSTR no longer works in a pipe. FINDSTR is not special; the same thing (namely nothing) happens with various versions of GREP and anything else used in a pipe.. On Win7/32, some (not all) programs used in the pipe cause conhost.exe to crash with a message (or just disappear).
v:\> echo foo | findstr foo foo v:\> type buffertest.btm set oldbuffer=%@consoleb[-1] set newbuffer=%@consoleb echo %@consoleb[%oldbuffer] echo %@fileclose[%newbuffer] v:\> buffertest.btm 19 0 v:\> echo foo | findstr foo v:\>