Excessive reallocation of buffers during HW-accelerated MPV playback

I'm using mpv on Fedora 31 with Vulkan and VAAPI HW acceleration, and finding that libplacebo seems to be responsible for very excessive reallocation of frame buffers/slabs during playback. I initially experienced this as a significant memory leak due to bugs in the Sway compositor (https://github.com/swaywm/sway/issues/4662) however this has been fixed and the issue persists.

Verbose mpv logs show:

[statusline] AV: 00:00:02 / 03:47:00 (0%) A-V:  0.016 DS: 2.220/5 Dropped: 4
[vo/gpu/vulkan/libplacebo] Allocating 5160960 of imported memory from fd: 37
[vo/gpu/vulkan/libplacebo] Allocating 5160960 of imported memory from fd: 37
[vo/gpu/vulkan/libplacebo] Freed slab of size 5160960 from imported fd: 37
[vo/gpu/vulkan/libplacebo] Freed slab of size 5160960 from imported fd: 37
[statusline] AV: 00:00:02 / 03:47:00 (0%) A-V:  0.015 DS: 2.235/5 Dropped: 4
[vo/gpu/vulkan/libplacebo] Allocating 5160960 of imported memory from fd: 37
[vo/gpu/vulkan/libplacebo] Allocating 5160960 of imported memory from fd: 37
[vo/gpu/vulkan/libplacebo] Freed slab of size 5160960 from imported fd: 37
[vo/gpu/vulkan/libplacebo] Freed slab of size 5160960 from imported fd: 37
[statusline] AV: 00:00:02 / 03:47:00 (0%) A-V:  0.016 DS: 2.231/5 Dropped: 4
[vo/gpu/vulkan/libplacebo] Allocating 5160960 of imported memory from fd: 37
[vo/gpu/vulkan/libplacebo] Allocating 5160960 of imported memory from fd: 37
[vo/gpu/vulkan/libplacebo] Freed slab of size 5160960 from imported fd: 37
[vo/gpu/vulkan/libplacebo] Freed slab of size 5160960 from imported fd: 37
[statusline] AV: 00:00:02 / 03:47:00 (0%) A-V:  0.015 DS: 2.245/5 Dropped: 4
[vo/gpu/vulkan/libplacebo] Allocating 5160960 of imported memory from fd: 37
[vo/gpu/vulkan/libplacebo] Allocating 5160960 of imported memory from fd: 37
[vo/gpu/vulkan/libplacebo] Freed slab of size 5160960 from imported fd: 37
[vo/gpu/vulkan/libplacebo] Freed slab of size 5160960 from imported fd: 37
[statusline] AV: 00:00:02 / 03:47:00 (0%) A-V:  0.014 DS: 2.241/5 Dropped: 4
[vo/gpu/vulkan/libplacebo] Allocating 5160960 of imported memory from fd: 37
[vo/gpu/vulkan/libplacebo] Allocating 5160960 of imported memory from fd: 37
[vo/gpu/vulkan/libplacebo] Freed slab of size 5160960 from imported fd: 37
[vo/gpu/vulkan/libplacebo] Freed slab of size 5160960 from imported fd: 37
[statusline] AV: 00:00:02 / 03:47:00 (0%) A-V:  0.014 DS: 2.255/5 Dropped: 4

Repeated throughout playback. The buffer size is an even multiple of the video size/bit depth so I assume they are frame buffers. This issue only occurs with VAAPI-accelerated playback and is not seen with SW decoding.

My system:

X1 Yoga Gen 4 (i7-8665U w/ UHD 620 iGPU)
Fedora 31 beta
Sway/wlroots Wayland compositor (-git)
Mesa 19.3-git (using new Iris driver and intel-media-driver although issue also seen with old i965 driver/libva driver).
Kernel 5.4rc3
mpv-git

mpv.conf:

fs=yes
hwdec=auto
vo=gpu
gpu-api=vulkan
video-sync=display-resample
profile=gpu-hq
interpolation
tscale=oversample

I've discussed this with the mpv Wayland developer (https://github.com/swaywm/sway/pull/4667#issuecomment-545521941) who suggested it is probably a driver issue but it does occur with both the i965 and new Iris drivers so I thought I would mention it in case this was a libplacebo issue. Thanks!