My experience until earlier this year was very similar (albeit with a sample size of only around 100 entries), but as of around February they simply stopped doing it. I even had to escalate it to a supervisor once and he told me that their new policy was not to require a passport stamp if the visitor has received leave to enter (aka a previous stamp) within the previous 6 months and was entering to resume the same purpose within the expiry period. They will continue to stamp in the case of special conditions being attached to a stay (eg. leave to enter for a period other than the default 6 months, VIPP notations, IT notations, etc...) but otherwise it is at the discretion of the officer. As a result, I am only 1 for 6 this year with Heathrow passport stamps after missing only 1 in the previous 20+ years (which was due to a flight diversion when they simply waved everyone through en route to hotels).