Hi, find the bug:
I have built many underground belts, and they work fine, but one of them sends items non stop into the buffer. When I deleted the exit point, I have over 40,000 items returned into my inventory. Tried to delete it, and rebuild it using different speed belts, tried to send different items, but it still goes non stop.
Sorry for the wrong title, I thought it is due to using different belts, but after that I realized it has nothing to do with belt speeds.

Are you running mods that change stack sizes? Can you please upload your save game?

I do use some mods, yes, but as I said only 1 belt is doing this. How can I upload save game file?

Here, I uploaded it to my google cloud:
https://drive.google.com/file/d/1mKUDlMXSrqljlFeY25KmGpiw10or36p1/view?usp=sharing
Dodgy belt is the one which moves Soil (Pyanodons mod) from North to the South.

By the way, last time you updated your mod, I was getting error launching it, I think you accidentally misspelled Beltlayer as Belflayer in the code somewhere, which I renamed and it worked again.

You can find your saved games according to instructions here:
https://wiki.factorio.com/Application_directory#User_data_directory
Then upload it to a file sharing site like Dropbox or similar, and post a link here.
Alternatively, make a forum post on the Beltlayer thread and attach the save: https://forums.factorio.com/viewtopic.php?t=62567

Hmmmm. That's more mods than my computer can load at the same time. Factorio wants 11 GB of RAM and I only have 8.

Which connector is giving you trouble? The soil connector right next to your character's position? That one only appears to have a few hundred items buffered.

That's because I laid new belt and saved the game. Speed up game and see, belt swallows all the soil non stop, but at the exit soil moves slowly. You can even delete a tile on the belt feeding soil inside the factory, soil intake never stops.

Could you also provide your mod-settings.dat? I think you've changed quite a few settings from the default, which aren't included as part of the save game.

I waited long enough at speed x64 and belt stopped. I deleted the the belt and got 40,008 of soil back into inventory.
Soil stack is 10,000, so I got 4 stacks.
Here is dat file:
https://drive.google.com/file/d/1eAZSPN3fvgcknli_Ww_QMHFNaF0JidWh/view?usp=sharing

This belt is no different than others, I was mistaken. Every belt gives 4 stacks back, just tested with other items.
If Stack is 1000, I get 4000 back,
if stack is 3000, I get 12000 back.
So I thought it is only this one is buggy, because Soil stack is 10,000, so the belt eats 40,000 as a buffer, hence I noticed this one as it took ages to fill it up full.
I know my stacks are big being x10, but is it right that the buffer is 4 stacks on your mod?
And it is 4 stack on both ends, so belt stops taking any items only after 8 full stack. That is a lot of stuff. Very hard to sort out bottlenecks with such a big buffer.

The default setting is 2 stacks per buffer chest, and there are 2 buffers per connector, one above ground and one below ground, so yes, 4 stacks is correct. You can change the stacks per chest to 1 in the mod settings, which would make it "only" 20,000 soil.

Why 4 stacks? Because default stacks for ore are 50 items, and blue belts move items at 40/second, so in the 5 seconds (by default) between transfers, that's 200 items. Come to think of it, it should probably be 4 stacks per chest, for a total of 8 stacks.

Since everything is adjustable by settings, you can adjust for most mod setups, but there's not much I can do about this issue when using 10x stacks with items that by default already stack to 1,000.

In the latest version, you can change the mod setting to "Show buffer contents." That will let you open up the buffer chests and give them a different limit (via the red X). That should help, but if even 1 stack of soil is 10k, that's still the minimum.

Thanks Therax