(Windows) Problems with processes on master branch

[Óscar Fuentes]

With a fresh MSYS2/Mingw-w64 build executed on Windows 11 64bits with
-Q, emacs mangles the output of processes and does not detect when they
end.

For instance, with emacs -Q

M-x shell

it shows

icrosoft Windows [Version <omitted rest of output>

Note the missing initial M. Then, emacs hangs (no input possible, C-g
doen't work) showing (Shell:run) on the modeline.

If I execute eshell instead of shell and invoke an executable like

$ gcc -v

the shown output misses the first line:

Using built-in specs.

and then eshell keeps waiting as if the command would still be
executing, which is false (Windows' Task Manager confirms that there is
no gcc executable running).

Is this a known problem?