Understanding Water Accumulation in Sink's Inner Basin: Overflow Hole & Drainage Issues

I have a problem with water in the sink. It's not the water that collects in the basin when the plug is closed, but the water that collects in the inner basin [brown area].

I noticed the problem when removing the sink when the sound of splashing water came out of it.
When I tilted it back, water started to flow out of the holes on the back (not the holes for the mounting screws) [blue arrows]. It was a lot, over a liter. Generally, the sink functioned like a water container.

During the inspection of the washbasin, I noticed that water enters its internal chamber through the overflow hole in the wall of the washbasin. Previously I thought there was only one outlet from this hole down to the plug [green arrow]. However, I checked that in addition to the down channel, there are two more channels leading diagonally from the overflow hole towards the upper corners of the sink to the chamber in its center [pink arrows].
Under special conditions, such as intensive clogging of the washbasin, water flows from the siphon through the plug and drain channel into the chamber where it collects.

Under normal conditions, when the sink is hanging on the wall, it is unobservable, except that a slightly unpleasant smell comes from the area of the overflow hole.

The problem has nothing to do with a clogged cork or siphon, because the sink is currently removed and devoid of all connections and I am able to pour water into its inner chamber through the overflow hole. This water is not able to get out of there in any other way than by tilting the sink backwards and waiting until it pours out of the chamber through these two holes.

I have a question, is it normal for sinks to take on water inside? Or is something clogged in it and the water is not able to flow down to the cork on its own? I don't like the fact that water collects there, which after a long time probably becomes a living environment for many creatures.

Water flows from the overflow hole to the hole at the plug, i.e. the green channel is unobstructed.
In the place of the green arrowhead, I see water flowing from the overflow hole.
When I put a hose with water to the overflow hole, by pouring water downwards in accordance with the green arrow, the water flows down without any problems.
When I pour up according to the pink arrows, I fill the sink from the inside.
In my opinion, it should not be possible to fill the sink in such a way.

Either this design or some manufacturing defect inside. Anyway, why pour water up through the overflow hole? If it bothers you that much, then flood the space with something that will harden and will not allow water to pass through.

Good idea with the foam.
Earlier I thought about silicone, but it's harder to spread in such a place.
The only question is whether the clogging of channels that have been made for some purpose will not disturb the function of the outflow.

mkk5000 wrote:

Good idea with the foam.
Earlier I thought about silicone, but it's harder to spread in such a place.
The only question is whether the clogging of channels that have been made for some purpose will not disturb the function of the outflow.

Yes, this will cause the sink overflow to malfunction.
And to be honest, the description shows that the overflow is not working properly, and most likely the reason is the use of the wrong part under the washbasin plug.
This sleeve-pipe with a flange inside the washbasin, threaded at the bottom, must have holes in the spigot part (the one inside the washbasin) that allow the water flowing through the overflow hole to drain away, bypassing the clogged plug.

I sealed the upper channels in the overflow with silicone. Despite this, the transfer works fine. So the topic is no longer active. Thanks for the replies.