I was just fishing to see if it was possibly a common issue. Had one to lose bolts on rear trunion. Remounted the axle and replaced steer lines as these were destroyed. Installed proper bolts and lockwashers. Used blue thread locker and torqued to specs. Was not a fun job at all as the lift was in a tight space inside the mfg plant and could not be fished out due to weight issues and not another lift of equal or greater capacity to lift with. Was a pain but i and another tech somehow pulled it off.

I usually see this in operations where someone is lifting more than the capacity of the lift, and the steering wheels/tires are coming off the floor, since weight is what usually keeps the steer axle trunion in place, and the bolts only get stressed when the weight comes up off the axle, leaving the axle hanging by the bolts.
always checking that the wheel steer stops are hitting and what stops the axle from turning too far on each PM is always a 'must' (as it takes almost no time at all to eyeball the stops and make sure they are being hit) too.

i have seen this happen on occasion on various lifts over the years.
many things can cause this from incorrect torque from factory to terrain the lift operates on jolting or vibrating the steer axle or frame either loosening or breaking bolts, maybe even issues with the rubber bushings holding the axle in place or all of the above.

I would evaluate the axle , check the bolts, make sure they had proper lock washers on them and then find out the specs on the bolt torque and re-torque them. If the problem persisted then go back and take a closer look at the mounting etc., Figure out why they keep coming loose. Maybe the bolt is too long bottoming out and not tightening up correctly? Lock washers getting squashed, becoming misformed when compressed and rendering them ineffective? Could be anything causing it.

I'm not familiar with that model, but the Doosan B18T-5 is prone to having the steer gear break at the front mount. It's a design problem. Had 4 steer gears break, non fixable. The replacement ones were beefier at this area. The bolts came loose after the gear cracked.

Is there any type of locking on the bolts?

Have you tried Nordlock washers?

I have not seen that either but I have seen the steer cylinder come loose or shear the bolts off.

Thanks for your 2 cents Prentice.

I have not seen it on a doosan but..............yes I have seen it happen on other equipment. The axles take a lot of abuse