Even a couple of years ago, i saw on the forum that people were refused gcb for only returning one key. That is why, I zip tied both keys together a few days before handover.
On handover, the garage looks at your old car and ticks the good condition on the computer. It is decided then and there if you get gcb. Only one key, no gcb.
Yes quite recently we have seen people refused extensions over the 3 years, unless its considerably adapted and low miles.
The lease systems and checks have changed. They may change again, who knows.
Your experience just re enforces the current way the system is run. Irs nothing personal, it is what it is.