HEVC video generated by OpenCV/ffmpeg shows black screen. Bug?

[guju]

Recently I needed to edit a HEVC video that was generated
by standard HEVC output of OpenCV (ffmpeg 5.0.1).

Opening gives no error message but only a black screen.
(Opening it with VirtualDub2 (2020), ffmpeg CLI or MPC-HC: no problem).

Can you tell what is going wrong here?
Is it a bug of ADM? (Or maybe of HEVC-type generated?)

https://c.gmx.net/@1155842887640945833/bJH2qPQ-Tm279HnSREF4SA

EDIT:
It is lossless HEVC.

[Geo_log]

what is going wrong here

- container .avi
Save the output of OpenCV to .mp4 container. 

[guju]

Thank you, HEVC in MP4 is accepted by ADM.

However who's the culprit?

Does OpenCV writes HEVC in MP4 errornous?
(There would be a bug in OpenCV)
Is ADM unable to read HEVC in AVI?
(That would be bug or restriction of ADM)

[eumagga0x2a]

Avidemux currently doesn't understand that codec tag "hev1" means HEVC.

(More precisely, only the MP4 demuxer supports that tag. This is why a placeholder, invalid decoder is chosen when opening this video stored in an AVI container, while an MP4 works.)

[eumagga0x2a]

Tag added, please try a future nightly.

[guju]

That's fine, I'm glad for my little contribute to enhance ADM.
Thank you! 

[guju]

Out of curiousity I tried saving hevc/ x265 in avi containter with ADM.

==> "Bad idea. H264/ H265 in AVI... better use MKV".

Is that still up to date, or just a relict?