chcp command changes the piping behavior

There is a script cat.bat emulating unix cat: it reads from input stream and then write into output stream:

@echo off
for /f "tolens=1* delims=:" %%a in ('findstr  /n "^"') do (
  echo(%%b)
)

I can use it in two manners with the same result:

type textfile | cat.bat
cat.bat < textfile

If I insert line chcp>nul before for loop then only the first form (with |) works. The second one (using <) outputs nothing.

Why could this be? And how to achieve unified behavior?

It's a strange behavior, I can't explain it, but I can circumvent it.

Using find instead of findstr solves the problem.

@echo off
chcp > NUL

setlocal DisableDelayedExpansion
for /F "delims="  %%a in ('find /n /v ""') do (
    set "line=%%a"
    setlocal EnableDelayedExpansion
    set "line=!line:*]=!"
    echo(!line!
    endlocal
)

I suppose the chcp simply flushes the input stream, but only when the input comes from a redirect. But FIND has the special ability to reset a file redirection, each time it's called.

test.bat

@echo off
chcp

echo Read from stdin with findstr
findstr  /n "^"
echo(
echo Read from stdin with find
find /n /v ""

Test with: type text.txt | test.bat

Aktive Codepage: 850.
Read from stdin with findstr
1:Line1
2:Line2

Read from stdin with find

Test with: test.bat < text.txt

Aktive Codepage: 850.
Read from stdin with findstr

Read from stdin with find
[1]Line1
[2]Line2