The stencil buffer doesn’t draw a part of the tiles - c++

Question

Well, I’m trying to draw tiles into a stencil buffer, but while drawing obviously happens something that I don’t understand and during the drawing disappearing (not drawn) part of the tiles.
So, how I draw:
// Enable blending.
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
// Enable testing.
glEnable(GL_STENCIL_TEST);
// Disable depth test.
glDisable(GL_DEPTH_TEST);
// Set stencil buff to 0.
glClearStencil(0);
glClear(GL_STENCIL_BUFFER_BIT);
// Here I got all visible tiles by a camera.
auto const visible_tiles = camera.visible_tiles();
// Draw tiles into a stencil.
glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);
glStencilMask(0xFF);
// Don't output the color.
glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
// ...
// Here I had a loop where I go through each tile from all which was visible for the camera.
auto tile_id = 1;
visible_tiles.for_each_tile([this, &tile_id](auto const& tile_position) {
// Output tile ID into stencil buffer. I assume there will never be more than
// 255 tiles.
glStencilFunc(GL_ALWAYS, tile_id, 0xFF);
mesh_->draw(); // << Here I draw.
++tile_id;
});
// Disable testing.
glDisable(GL_STENCIL_TEST);
However, how my problem looks like?
]1
If I disable stencil testing at all, everything is OK.
And we draw it correctly.
UPD: With the help of the debug, I managed to narrow the circle of suspects )))
And I realized that the problem was somewhere in glEnable(GL_STENCIL_TEST), namely when I repeatedly call my method “draw(…)”.
So, I just did the following:
static bool flag = false;
if (!flag) {
glEnable(GL_STENCIL_TEST);
flag = true;
}
So, now I call glEnable(GL_STENCIL_TEST) only once, at first call of draw, and didn’t call glDisable(GL_STENCIL_TEST);
And it looks as if everything works correctly, at least now I didn’t see any defects.
But why this works?