Fix: Use correct custom field id when splitting

[shamoon]

Proposed change

Closes #6297

Type of change

• Bug fix: non-breaking change which fixes an issue.
• New feature / Enhancement: non-breaking change which adds functionality. Please read the important note above.
• Breaking change: fix or feature that would cause existing functionality to not work as expected.
• Documentation only.
• Other. Please explain:

Checklist:

• I have read & agree with the contributing guidelines.
• If applicable, I have included testing coverage for new code in this PR, for backend and / or front-end changes.
• If applicable, I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• If applicable, I have checked that all tests pass, see documentation.
• I have run all pre-commit hooks, see documentation.
• I have made corresponding changes to the documentation as needed.
• I have checked my modifications for any breaking changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.12%. Comparing base (ae2b302) to head (9fc7a76).

❗ Current head 9fc7a76 differs from pull request most recent head a7a7d69. Consider uploading reports for the commit a7a7d69 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #6303   +/-   ##
=======================================
  Coverage   97.12%   97.12%           
=======================================
  Files         420      420           
  Lines       16917    16917           
  Branches     1286     1211   -75     
=======================================
  Hits        16431    16431           
  Misses        486      486           

Flag	Coverage Δ
backend	95.94% <ø> (ø)
frontend	98.56% <ø> (ø)

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