This appears to be a behavior change in macOS 27, and it's one not documented by Apple. Their instructions to add an item to the reading list still indicate one should click the (+) button to add the page to the Reading List.
More info from Apple: Keep a Reading List in Safari on Mac – Apple Support
The behavior is context-dependent, and I strongly suspect that Apple Intelligence is 'helping' to decide whether the page belongs in a Bookmarks folder or in the Reading List. For me, with an Apple-related page the button/field points to one of my Bookmarks folders (where there are other Apple-related bookmarks), but if the page is something unrelated to my bookmarks, then the option points to the Reading List. A plumbing repair search results page, for example:

As a workaround, when you click on the (+) button, don't use a short click-and-release. Instead, once the (+) button appears after hovering the cursor over the Smart Search Field, either secondary click (right click, control click) on the (+) button or simply hold the click for longer than normal. Either way, a contextual menu will appear and allow you to select Reading List as the destination.

Only Apple can revert to the pre-Golden Gate behavior. Given the broader role they are giving to Apple Intelligence, I am not sure that such a reversion is likely, but to have Apple consider your feedback, submit it here:
Feedback - macOS - Apple