Issue with caching and build arguments

[orf]

Actual behavior
Building two images with differing build args results in incorrect images being built, when caching is enabled.

Expected behavior
Two individual images should be built

Reproduction
We have the following reduced Dockerfile:

FROM our_base_image
ARG python_version
RUN pyenv install ${PYTHON_VERSION}

If we build this Dockerfile with Kaniko:

kaniko/executor --build-arg PYTHON_VERSION=3.6 --destination=OUR_ECR_REGISTRY/python:3.6 --cache=true --cache-repo=OUR_ECR_REGISTRY/kaniko

and at the same time:

kaniko/executor --build-arg PYTHON_VERSION=3.7 --destination=OUR_ECR_REGISTRY/python:3.7 --cache=true --cache-repo=OUR_ECR_REGISTRY/kaniko

Then there appears to be some form of cache collision. The python:3.7 image has python 3.6 installed. To be clear: two different images are pushed.

We are using the latest kaniko docker image in Kubernetes. Could this be related to #639?