Well, now that you mention it... maybe this is where I'm running into troubles.... seeing how there aren't any ribs in there. This grid only shows up horizontally as an inch (or so) wide strip halfway up the long side of the negative. When I curl the negative and screen and slide it into the tube, I do feel like the fact that the neg wants to uncurl puts a lot of pressure at the same point where the grid ultimately shows. Now I'm wondering if it's creating so much pressure that the presoak can't get in there and clear that antihalation layer. It's clear everywhere else.
I may switch to presoaking in trays and see how that works. Thanks for the tip... even if it was inadvertent!
As for developing, I only develop one sheet at a time in the tubes. It's not speedy, but it's all I've got. One of these days I'd like to make my own tubes, but the time just hasn't presented itself yet. Is this what you meant?