I think that the 20 minute rule considers the total time spent for a piece of goods from origin to destination and that includes:
- Time from industry to platform in origin (you can see that packets actually “walks” from industry to platform if you turn display of cargo packets on)
- Time waiting for the train to arrive.
- Travel time.
- Time from platform in destination to industry.
So you have to be well under 20 minutes in line frequency to fulfil the requirements. For instance, a line with a frequency of 19 minutes, accounts on average for a 9 and a half minute wait at origin platform. And you have to add times in items 1, 3 and 4.
The game probes the chain and when it sees that you aren’t reaching the targets in time, it gets cut off. Or at least I understand it that way.
Another source can be that if you have automatic replacement turned on, cargo is lost when replacing and that counts towards cutting off a line.