It looks like your workflow is using version 1 of the Imputation tool.
This issue was fixed in version 2 of the Imputation tool which was released several years ago. I've confirmed the fix in version 11.7 (pre-2018.1).
Version 3 of the Imputation tool was just released in 2020.2. It fixes several issues in version 2.
To update an Imputation tool in an existing workflow, right-click the tool and select version you what to use. Then click back on to the tool and confirm the configuration.