gnus-thread-operation-ignore-subject

gnus-thread-operation-ignore-subject is a variable defined in `gnus-sum.el'.
Its value is

t

Documentation:
*If non-nil, subjects will be ignored when doing thread commands.
This affects commands like `gnus-summary-kill-thread' and
`gnus-summary-lower-thread'.

If this variable is nil, articles in the same thread with different
subjects will not be included in the operation in question. If this
variable is `fuzzy', only articles that have subjects that are fuzzily
equal will be included.

You can customize this variable.