I know it sounds like a setup issue, there is no turbo: false on any parent node and I can see that turbo is loaded correctly by checking window.Turbo on the console.
I made it work by adding data: {'turbo-method': 'get', 'turbo-frame': "my-frame"} directly on my <a> tag.
One possible relevant issue is that this link is present on a div that is rendered via stimulus reflex (its an older app so reflex will be working alongside turbo for a while) but I fail to see how that is relevant since the DOM structure is correct.
How should I debug this to understand why turbo is performing a full page navigation instead of a visit?
have you made sure the frame_tag ‘my-frame’ actually exists on the page. It fequently catches me out since it seems you wont get a turb request unless the frame exists.
One way to test this out is to use data-turbo-permanent element with an id and put some content inside of it on one page then navigate to another page where you have an element with the same id but different content. if the content stays the same as the first page then you will know that Turbo is working correctly!