fix preview: custom header with two months displayed

[navedqb]

Fixes #5677

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.14%. Comparing base (bb8049c) to head (1179ac9). Report is 41 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5686   +/-   ##
=======================================
  Coverage   97.14%   97.14%           
=======================================
  Files          30       30           
  Lines        3364     3364           
  Branches     1420     1420           
=======================================
  Hits         3268     3268           
  Misses         94       94           
  Partials        2        2           

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