1. COFFEE IS BEING GROUND TOO FINE.
Most coffee makers come with a filter or strainer that captures the ground coffee. This prevents any of these grounds from making it into your final product. When the hole size in your coffee strainer/filter is larger than the finely grounds, they will go through.
Here is Google.
If the coffee is very finely ground, a dense lump may be formed
If your coffee is very finely ground (such as for espresso), it may form one dense lump. This causes a build-up of water that will pool and overflow. When the water overflow, it will contain coffee grounds that end up in your carafe.
SOLVE: Don’t use a too fine grind. Look for coffee that has been specially ground for your coffee grinders methods, if you’re grinding your own coffee beans, know exactly the level of grinding coffee.
When using drip coffee makers, using plastic or metal filters without paper, sometimes fine coffee grounds can still pass through. However, there usually aren’t so many that they make drinking the coffee unpleasant. If this situation bothers you, add a paper filter.
2. PROBLEMS WITH PAPER FILTERS.
When you add a paper filter to your coffee maker, it sometimes does not stick to the walls of the basket. If you are brewing your coffee with a grind that is too fine or using more than enough coffee, this will cause a build-up of water that overflows into the gap between the paper filter and the filter basket. This causes a lot of ground coffee to bypass as there is absolutely no filtration from the filter basket, resulting in a very muddy cup of coffee.