Bitmap transparency on semi-round 16-color watches

Hello! Quick question regarding bitmap transparency on watches with semi-round 16-color displays — uh, does it work?

I'm building a watch face that uses a bitmap image with a transparent background, which works fine on devices with 8-color and 64-color screens. However, on 16-color devices (which all happen to be semi-round), the bitmap renders with a black background instead. It's being included via an XML layout, and I've also tried adding a custom color palette that explicitly sets transparency to enabled — the palette restriction was clearly visible, but transparency still didn't happen.

For completeness, here's a selection of devices I've simulated on:

  • Forerunner 45: CIQ 1.4, 8-color screen, yes transparency
  • Forerunner 630: CIQ 1.4, 16-color screen, no transparency
  • Forerunner 735xt: CIQ 2.4, 16-color screen, no transparency
  • Approach S60: CIQ 2.4, 64-color screen, yes transparency

Am I missing a CIQ development concept or a documentation note somewhere?