Build fails with `ERROR: failed to walk` if buildkit is on

Prerequisites

OS Ubuntu 18.04.5 LTS, Linux 4.15.0-142-generic x86_64 GNU/Linux and/or
OS Ubuntu 20.04.2 LTS, Linux 5.4.0-72-generic x86_64 GNU/Linux
docker version 20.10.6, build 370c289
containerd containerd.io 1.4.4 05f951a3781f4f2c1911b05e61c160e9c30eaa8e
runc version 1.0.0-rc93 12644e614e25b05da6fd08a38ffa0cfe1903fdec

Issue

I run multi-stage build in the loop with cache clean (docker system prune -a -f --volumes) after each build, and SOMETIMES build fails with the error:

#4 [1/5] FROM docker.sd.datadvance.net/sd/pseven/pseven-2k/base-ubuntu:73950f029accb232c84544962b1ea85614cb7af4901371f5be0793f714f69595
#4 sha256:0c5922668a6334173091e103bfe19c213c374cc726ae41ad177d1e0d5788df85
#4 DONE 0.0s

#5 [internal] load build context
#5 sha256:00ba4790367f93ee18014b48b3352d94e0e8067b8534f25fb0a53ed77d45f7f2
#5 transferring context: 2B done
#5 DONE 0.1s

#4 [1/5] FROM docker.sd.datadvance.net/sd/pseven/pseven-2k/base-ubuntu:73950f029accb232c84544962b1ea85614cb7af4901371f5be0793f714f69595
#4 sha256:0c5922668a6334173091e103bfe19c213c374cc726ae41ad177d1e0d5788df85
#4 CACHED

#6 [2/5] COPY ./buildui/curlrc /root/.curlrc
#6 sha256:7318eaffbad4a45b276670c5ce0ebd53f00e9515e7c62d08e37bfe36c893949e
#6 ERROR: failed to walk /var/lib/docker/tmp/buildkit-mount588371557/buildui: lstat /var/lib/docker/tmp/buildkit-mount588371557/buildui: no such file or directory

#4 [1/5] FROM docker.sd.datadvance.net/sd/pseven/pseven-2k/base-ubuntu:73950f029accb232c84544962b1ea85614cb7af4901371f5be0793f714f69595
#4 sha256:0c5922668a6334173091e103bfe19c213c374cc726ae41ad177d1e0d5788df85
#4 DONE 0.2s
------
 > [2/5] COPY ./buildui/curlrc /root/.curlrc:
------
failed to compute cache key: failed to walk /var/lib/docker/tmp/buildkit-mount588371557/buildui: lstat /var/lib/docker/tmp/buildkit-mount588371557/buildui: no such file or directory

ones per 5-20 build, the build fails not on this particular step each time, it can be a different one, but with the same error. If buildkit is turned off, 168 builds were finished without any problem. I have checked the issue on several machines but got the same result.

I have tried to extract some docker build part from the project as an example. It is not exactly what I am running, because the original build contains a huge amount of steps and the project sources, but I hope it helps:
DA-baseimages-example-20210504.zip

Do you have any idea why this could happen?

Start with the attached DA-baseimages-example-20210504.zip and reproduce the multi-stage build with BuildKit enabled, especially the COPY ./buildui/curlrc step. Compare repeated builds with cache cleanup against builds with BuildKit disabled, and inspect the intermittent failed build-context walk. Done means the failure is reproducible and its cause and reliable resolution are documented or covered by a regression test.