Simple test - new project, put a couple of buttons on the form via the form designer - no change from my previous, no indication of focus on the buttons. Strange given the message dialogs and open/save dialogs behave as I want them to.
I've got a copy of lubuntu on this test laptop and, if I've time tomorrow, I'll stick lazarus on it and see what happens. Also have a play with TBitBn. Also see if anything changes with any of the other stock cinnamon themes. If I can't get a result on this, I'll have to disable tabbing and rely on the cancel and default properties for keyboard input on Tbuttons.
EDIT - it's a function of the controls setting in the theme, my default mint-x doesn't work (or any colour variant of mint-x). Changing to either adwaita or mint-y for the controls works - focus is shown on the tbutton. Not that you would expect it to, but played around with the desktop (linux mint/cinnamon/mint-y dark) and default icons - no change.
Sort of solved in the sense that the target for my app will be a lightweight distro booting off a usb stick - it won't be mint, so this problem should go away. Would still like to know why so I'll still have a play with TbitBn.