I think you are wrong about that.
In many cases, the test procedures are even written before the actual feature is build.
Much effort is put into regression tests, simply because they are extremely important and reduce the risk of having severe bugs send to the beta team.
However, these regression tests are only testing the functionality of the feature, and other regression tests (from other features and within other places of the DAW) are making sure that the new feature doesn’t break anything else. This does not include “cosmetic” bugs. Which is the case of the problem you are referring to.
The trim function works as designed, the only thing that was “broken” was the delta indicator. Which is the visual aid for the feauture. Which again, by itself is located in another area. 'Cause it was also used for midi tracks.
Not trying to minimize “your” bug, but cosmetic stuff is usually last in the row of “things to be fixed”.
Will be fixed in the upcoming 6.5.2 by the way …
Fredo