This isn't really a mystery. We know the fossil record is incomplete, something like 99% of species are almost certainly not represented in the fossil record and what is present will massively be biased towards marine life.

Plants don't fossilize particularly well anyways so it's not surprising that we don't have records for these organisms. The fact that we have records for the existence of ferns at all is probably evidence of them suddenly becoming massively prolific (I wonder if some major environmental change occurred around that time ☄️) rather than them springing out a thin air.