Fix an issue with endless loops in Fn::Sub

[kddejong]

Issue #, if available:

Description of changes:

• Fix an issue with endless loops in Fn::Sub

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.34%. Comparing base (2cc2226) to head (cd622b0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3503      +/-   ##
==========================================
- Coverage   93.35%   93.34%   -0.02%     
==========================================
  Files         325      325              
  Lines       11089    11086       -3     
  Branches     2328     2327       -1     
==========================================
- Hits        10352    10348       -4     
  Misses        414      414              
- Partials      323      324       +1     

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