It's not really "fixed" in FotRSU KaleunMarco, though CapnScurvy did make some of the "guilty" ships "noisier" so that they are easier to hear... As Bilge_Rat points out, if the game can "see" the other ship, it will eliminate the sonar lines, even if your periscope is not up. Sort of like the XO has the other scope up, "helping" you out... so go just a bit below PD, and you should get the sonar lines back. Shouldn't be that way, but... it is SH4... As for the coastal ships not making noise, those are Type=104, which are "silent" on purpose. Some sampans though, did have little diesels in them, so... go figure.
