I don't know how that happened. I deleted and rebuilt the tile and it seems to have solved the problem.
I believe I encountered the bug in 2.0.9 or 2.0.10 initially, but if I load the save in 2.0.11 I can still see this belt accumulation.
Screenshot & save attached.
![Image](https://forums.factorio.com/images/ext/9961501eeff0b589aedad1c194f06586.jpg)