HomePod and HomePod Mini differences

Both HomePod (2nd generation) and HomePod mini can act as Thread-enabled home hubs.

The full size Homepod can act as a Matter hub for compatible smart home devices and accessories, facilitating faster connections and response times.

So although both should work the same for HomeKit, the big 2nd gen homepod offers more advanced smart technology for certain smart devices requiring that. It might be the better choice and why it seems not to be working as expected. It really depends on the type of door lock and why one should read the specs on what it says you need. Not knowing which door lock this is, I can’t give a specific answer.

Having said all that Apple has this support documentation: Set up your HomePod, HomePod mini, Apple TV, or iPad as a home hub - Apple Support

• You must set up a home hub to add Matter accessories to the Home app.
• Thread-enabled Matter accessories require a Thread-enabled home hub, such as HomePod mini or Apple TV 4K (3rd generation) Wi-Fi + Ethernet, or a supported third-party border router.

It’s very confusing! Perhaps someone else can chime in with a better answer?