It's been a while, but I thought if share what worked for me.
Turns out my problem was a bad ro membrane in my filter. Once I replaced that and got tds back to 0 on the top off water, it was fairly easy. I did 5g water changes every day for a week, then 5g every other day for two weeks.
It's been a few months with no sign of returning. So, if/when you come up against this, just be diligent until it's gone. Pretty simple fix overall, just took some time and patience.