This isn’t really a bug, more a missing feature. Has been raised numerous times in the various main boards and Suggestions forums but to date no real response from DTG on the issue. Which given MSTS 21 years ago had this, along with different track noise crossing various types of bridges or level crossings, is a pretty poor state of affairs. It will become even more apparent in Spirit Of Steam blasting the Jubilee out of Lime Street with no reverb off the tight cutting or tunnel walls. Guess all the effort must have gone into the animated cows…
This is another "used to have" feature: it's present on Sand Patch. (Also, you get echoing as you go under overpasses).