AMD VAAPI AV1 doesn't reach configured bitrate in CBR-Mode on VCN5 (RDNA4)

Operating System Info

Other

Other OS

Arch Linux

OBS Studio Version

31.0.3

OBS Studio Version (Other)

No response

OBS Studio Log URL

https://obsproject.com/logs/ba0WLB02QJjGvf0n

OBS Studio Crash Log URL

No response

Expected Behavior

Bitrate should reach the configured bitrate when using CBR-Mode.

Current Behavior

Bitrate is unstable and regularly don't reach the configured Bitrate.

H264 / H265 works fine and behave as it should.

Steps to Reproduce

HW: Ryzen 7 5700X / AMD RX 9070 XT / Linux-zen 6.14.2 / Mesa 25.1.0-devel (git-fc7badeac0)

1. Video Encoder AMD FFMPEG VAAPI AV1
2. Rate Control CBR
3. Bitrate 50000 Kbps
4. Keyframe Interval 2s
5. Set Max B-Frames to whatever you like
6. Start stream
7. Observe bitrate fluctuations and don't reaching the desired Bitrate.

50000 KBit without B-Frames results in ~ 29000 KBit Average
50000 KBit with 1 B-Frame max. results in ~ 15000 KBit Average
50000 KBit with 2 B-Frames max results in ~ 22500 KBit Average

Anything else we should know?

Looks like the same Problem in a way reported here: https://github.com/obsproject/obs-studio/issues/12013

Start by reviewing the OBS log and reproducing the reported AMD FFMPEG VAAPI AV1 configuration on Arch Linux, then compare the related issue #12013. Verify behavior across 0, 1, and 2 maximum B-frames at 50000 Kbps; done means CBR output consistently reaches the configured bitrate without the reported fluctuations.