tonibat

Recumbents can help with back problems, but they can also can cause them.
The pressure you can impose on you back is much larger on a recumbent, for instance in sudden acceleration or at climbs. (The reason is simple: in a recumbent the tensions incurred by forced extension of your thigh is resisted by your back against the seat, and the low back bones are in the middle. By comparison, in an upright a forceful thrust with your leg cannot transmit to you lower back more tension than your body weight, since the whole body is lifted upwards in the limit).
So, even if posture and comfort much better when lying, the stress that you could impose on your lower back is larger in a recumbent.

My advice, always be careful of sudden, strong forces in a recumbent.
If you already suffer from back issues, get an electric drive to be able to ride smoothly, or quit riding altogeher (till it gets solved).