terragrunt: .terragrunt-cache fails to update with new source "...?ref=..."

I’m updating my terragrunt.hcl’s terraform { source = "..." } } value with new ...?ref=<commit> values as I make adjustments, but terragrunt isn’t successfully pulling the new commit from the git repo. Instead, it complains…

[terragrunt] [/Users/jake/git/codecommit.us-west-2/jn-tf/jakesys/regions/us-west-2/vpcs/dev] 2019/06/25 18:04:19 Running command: terraform --version
[terragrunt] 2019/06/25 18:04:19 Reading Terragrunt config file at /Users/jake/git/codecommit.us-west-2/jn-tf/jakesys/regions/us-west-2/vpcs/dev/terragrunt.hcl
[terragrunt] 2019/06/25 18:04:19 Cleaning up contents of download folder /Users/jake/git/codecommit.us-west-2/jn-tf/jakesys/regions/us-west-2/vpcs/dev/.terragrunt-cache/QM1-63WWHsRnztH6ooxLgsdRMGg/tieM-yGmC7c1rfHpXzt3T9uRVsA
[terragrunt] 2019/06/25 18:04:19 Downloading Terraform configurations from git::ssh://git-codecommit.us-west-2.amazonaws.com/v1/repos/jn-tf-mod?ref=86e6f62 into /Users/jake/git/codecommit.us-west-2/jn-tf/jakesys/regions/us-west-2/vpcs/dev/.terragrunt-cache/QM1-63WWHsRnztH6ooxLgsdRMGg/tieM-yGmC7c1rfHpXzt3T9uRVsA
[terragrunt] 2019/06/25 18:04:19 Hit multiple errors:
error downloading 'ssh://git-codecommit.us-west-2.amazonaws.com/v1/repos/jn-tf-mod?ref=86e6f62': /usr/local/bin/git exited with 1: error: pathspec 'master' did not match any file(s) known to git

[terragrunt] 2019/06/25 18:04:19 Unable to determine underlying exit code, so Terragrunt will exit with error code 1

The hash subdirectories under .terraform-cache are always the same:

git::ssh://git-codecommit.us-west-2.amazonaws.com/v1/repos/jn-tf-mod//vpc?ref=6e30e8a
.terragrunt-cache
└── QM1-63WWHsRnztH6ooxLgsdRMGg
    └── tieM-yGmC7c1rfHpXzt3T9uRVsA
        └── vpc
            ├── _data.tf
            ├── _init.tf
            ├── _vars.tf
            ├── main.tf
            └── terragrunt.hcl
git::ssh://git-codecommit.us-west-2.amazonaws.com/v1/repos/jn-tf-mod//vpc?ref=c7f82af
.terragrunt-cache
└── QM1-63WWHsRnztH6ooxLgsdRMGg
    └── tieM-yGmC7c1rfHpXzt3T9uRVsA
        └── vpc
            ├── _data.tf
            ├── _init.tf
            ├── _vars.tf
            ├── main.tf
            └── terragrunt.hcl

If I delete .terragrunt-cache entirely, then it’s able to pull the new commit reference. I’ve also tested to make sure it’s actually pulling the specified commit and not just HEAD.

terraform 0.12.2 (via brew) terragrunt 0.19.4 (via brew) git 1.22.0 (via brew) macOS mojave