Not all those who wander are lost
It's more likely caused by some blockage within the gravel or at the overflow such that the water isn't getting out as fast as it once was. As long as your bog liner is high enough to contain the rise, no problem. Water over the gravel will usually allow algae to grow on to, also limiting the rate of egress. The blockage will decay naturally but l'd still watch that the level does' continue to rise. Either unclog or make the egress wider/more voluminous. Plant roots, especially from those that are aggressive, is one cause of blockage.In the initial write-up the idea seems to be to have the water below the top of the gravel in the bog filter. In later pictures it appears the water is an inch or so deep. Has the idea changed or is this the result of settling without topping off the gravel?