Is the following intended behaviour, since 1.12.2?
Using the latest release 1.12.2-4.9.1.92

Could this be related to #1398 ?

I know what this bug is. In the config there is a number for "Routed Pipes: Max Detection Count". A single pipe is only able to search through a certain number of regular transport pipes before finding another router. Once it has finished, it will stop.

Note that this is a limit on the total count. Therefore, if the limit is 100 and a router has connections on two sides, it will search 50 pipes in each direction. The more connections you add the less distance it is able to go to establish a connection to its neighbor. This is why in your pictures you can see that it can go left OR right, but when you connect both, it does not work.

Fix this by adding more routers along the path. Please do not increase the limit in the configs too much, as that will drastically effect performance at some point.

This may also be related to #1398, but I'll look into it some more.

Note that this is a limit on the total count. Therefore, if the limit is 100 and a router has connections on two sides, it will search 50 pipes in each direction.

That is interesting.
But it starts all over again from pipe to pipe, right?

Maybe this explanation should be integrated to the config. Because right now it reads like "It searches in a 100 blocks radius"

So each pipe searches these 100/direction blocks in each direction?