Forums Bug Reports 6.31 toolbar icon background issue

v6.31 comes up with a new set of modern looking icons. I like them.
However, I run across an important difference in icon behaviour:

Adding a background "colour" (that is: a unicrome .bmp) to the toolbar's set of icons in v6.21, all icons use the "expected/intended" icon transparency.

6.21 "old" icons w/o background:

6.21 "old" icons with background:



Now, doing the same in v6.31 results in non-transparent "boxes" around the toolbar's default icons, which renders their look quite ...hum... unlovely.

6.31 "new" icons w/o background:

6.31 "new" icons with background:



Note: That last icon (in all pictures respectively) is a custom icon added on my own - to demonstrate that the issue is limited to the "default" icons only.

Am I missing something, is this by design, or should it be regarded as a "cosmetical bug"?