AVIF color grid without alpha grid looses alpha when decoded

[y-guyon]

This is an image encoded with avifenc --grid 1x2 (at cd03eef using ext/aom):

(extension is .jpg but it is an actual AVIF file)

The pattern is the following:

[primary item grid]
   ^       ^     ^
   |dimg   |dimg |
   |       |     |auxl
[color] [color]  |
                 |
   [alpha aux grid]
      ^       ^
      |dimg   |dimg
      |       |
   [alpha] [alpha]

Here is the same image manually edited to match the following pattern:

[primary item grid]
   ^       ^
   |dimg   |dimg
   |       |
[color] [color]
   ^       ^
   |auxl   |auxl
   |       |
[alpha] [alpha]

I did not find anything in ISOBMFF, HEIF and MIAF specifications regarding the invalidity of this pattern. Compliance Warden does not report more errors than for the image generated only with avifenc (even fewer errors actually because there is one less grid to complain about construction_method=-1 on a derived image item).

avifdec (at cd03eef using ext/aom) leads to the wrongly opaque following PNG:

There is the same issue with libheif: strukturag/libheif#708