Checklists: Copy / Paste is Unreliable

### Details:
We've received reports indicating that Copying and Pasting checklists is still unreliable.

Let's further investigate what might be causing this!

/cc @elibud (thank you Eli!)

Start by reproducing copy and paste with checklist content in the Simplenote macOS app and trace the behavior from the relevant checklist and clipboard entry points. Identify the cause of the unreliable operation, then verify that checklist copying and pasting works consistently in the affected cases.

Written by the indexing model from the issue text.