gnus-summary-gather-subject-limit

gnus-summary-gather-subject-limit is a variable defined in `gnus-sum.el'.
Its value is

nil

Documentation:
*Maximum length of subject comparisons when gathering loose threads.
Use nil to compare full subjects. Setting this variable to a low
number will help gather threads that have been corrupted by
newsreaders chopping off subject lines, but it might also mean that
unrelated articles that have subject that happen to begin with the
same few characters will be incorrectly gathered.

If this variable is `fuzzy', Gnus will use a fuzzy algorithm when
comparing subjects.

You can customize this variable.