If it's a bi-fold frame, any looseness in the stem lower hinge, the headset adjustment, and the frame hinge, can all feel the same; With the front brake applied and rocking the bike fore and aft, feel a slight clunking. Process of elimination:

Assuming the frame hinge is the standard kind with an over-center clamp that holds it closed, make sure it takes some effort to close it, I think perhaps 15mm of gap when the lever first starts to have force; On mine, I need closure to be hard enough that a bit difficult to open the lever to not have any looseness when I push the seatpost and stem in opposite directions and reverse. Note, above does not apply to "clamping" style hinge closures like on some of the pricer Dahons, not sure about Terns.

Next, check the stem latch the same way, if and only if, it is also an over-center style clamp, but I think it is not, I think yours has a wedge-latch closure, in which case you need to check procedure for adjusting.

Then check that the headset adjustment is good; On Dahons, that involved opening the stem latch and partly folding the stem down, loosen the side clamping bolt on the stem base, check slack on center large (10mm) bolt that adjusts headset, this should be just snug, like tightening it with the short end of the allen wrench as a handle, don't reef on it hard. Then bring handlebar stem up and latch, check that handlebars are aligned with front wheel, tighten clamp bolt at base of stem.