It is easiest to see in a gif.
https://gfycat.com/DimWateryAustralianshelduck
Before:
After:
As the third inserter places its item down, you'll notice the leading iron plate "jump" ahead slightly. This causes a belt to lose full compression and makes full compression by inserters impossible when this bug occurs.
A few observations:
-an inserter does not need to be placing an item onto a row of items for this bug to occur (the row of items can be on an otherwise uninteresting stretch of belt)
-The front item can "jump" ahead multiple times in a row, implying that a row of items does not need to be touching
-This only occurs with an item jumping ahead - never lagging behind, and therefor only occurs to the leading item
-While I have not been able to figure out why this bug occurs in a specific location, it nearly always seems to occur in the same location (until you change the timing on the belt)
