Steps to replicate:
Create a custom field, Test Checkbox: Alphanumeric, CheckBox with 1 option: label "I agree", value 1.
Create a profile including this field.
Include this profile on a contribution page.
Edit Contact A and check the "I agree" checkbox.
On the contribution page, click "Not demo test, or want to do this for a different person?"
Choose Contact A.
Expected result: Form populates with Contact A's data.
I have a fix for this, will create a PR & link to this issue.