It's weird. Sometimes, when I turn on the tab bar, boom, it's aligned perfectly at the top of the text box, and I can tab to my heart's desires. Other times, it shows up on the other side of the screen, and I have to drag it over manually. Intuitively, I click on that little magnet thing, thinking it will "stick" to the top of the text box, but it does nothing...most of the time. Sometimes it does what I think it's supposed to do and jumps to the top of the text box.
In order for the magnet to work the (full width) top of the frame or column must be visible and there has to be enough space to fit the tab panel.
I've only just realised this too. To be fair, Adobe's own instructions don't call it a "magnet" but rather "Snap above frame" - https://helpx.adobe.com/indesign/using/tabs-indents.html
This also works when the top of a table is visible/not visible.
However, I think it would work better if the panel snapped above the text that was being worked on, as opposed to the frame... time to head over to the Feature Request/Bug Report Form again!