I considered increasing the diameter of the bar (via duct tape or some other means) but I am wondering why this started happening in the first place.
May I ask, how long have you used this boat (hours if possible)?
Those fittings will loosen off with time as do the turnbuckles, (based on the sea conditions and use) A check with an allen wrench might all that is needed to ensure there is no slop in the fittings. You may also consider larger area washers (if they will fit) to backup the 4 bolts on the crossbars.
If this does not resolve the issue, machining down the crossbar brackets may be required to ensure that you can get a solid fit.
Duct tape will deteriorate and loose it's thickness as the heat melts out the adhesive eventualy.
If the 'buckles' loosen off over time, it will cause a twisting motion on the front crossbar. (Considering also the extra stress placed on the fore crossbar mounting bolts added to by the motion of the mast in normal use.)
Remember there is a relatively thin plastic hull that can get thinner as the fittings work against each other.
If I were in your shoes I'd take a set of calipers to the thickness of the deck at the bolt on points for the crossbars and check it against the hull thickness from a far less stressed area.
I know we try to be as careful about maint as we can. However, considering the potential stress that is placed on 4 bolts to hold the front of the boat together, it seems more carefull checks are needed
Tropical conditions may also make a difference here as plastic, and metal contracts and swells at different rates within a very hot sealed deck in the day, and a large temp drop at night (or even in the shade).
Also consider the fact that the flat black fittings (that I saw in the photo closeups), when salt had crystalized on them, make up the crossbar and aka system in this case. (Flat black is way more heat absorbing than semi-gloss or high gloss black).