Well, since folks use gravel or 100% perlite for a reservoir wick, I doubt a loose soil would impede the wicking of water, even in peat. Especially if water wicks to the top elsewhere.
I suspect something else is going on. Perhaps that side of your SIP is a lot warmer, or it's closer to the fan. Or maybe something is going on with the soil itself, like it didn't get fully mixed, or a big chunk of hydrophobic peat never really got moistened. I'm not sure what your biochar texture is like, and a big, dry chunk of that might be interfering?
But I doubt the soil is too aerated. Either way, I would dig in and investigate before reamending--you know, if it were me!