Since your default disk is using the
FILESYSTEM_DRIVER environment variable, make sure that the value of the variable is set to
public as your default disk. It looks like it might be using your
local disk instead.
You could also try changing this:
$upload = $request->imagem->store('instituicoes');
$upload = $request->imagem->store('instituicoes', 'public');
to force using the
public disk instead of whatever the default is.