Lateral distortion - how is this possible and how do I fix it?

So this is a set of block mold components printed on my Phenom. As a general rule I’ve been getting very good prints on the machine, but I’m seeing this specific type of distortion and I can’t tell exactly why. There’s definitely some peel force happening because I’m printing solid block molds unsupported - but for the most part they’re coming out perfectly… except for this distortion. The “wiggle” is happening in the vertical plane, so literally the only way I can think of that it could occur is that the build plate itself is shifting around during printing. Is that even possible? The build plate is screwed down very tight, but is it possible that the peel force is low enough for the print to stay stuck to the bed, but high enough that it’s actually moving the plate around during printing?

This photo is two halves of a mold printed with their backs adhered flat to the build plate, and their mating surfaces printed downward into the vat. The interior detail and the fit of the mated surfaces is very clean - almost perfect. The only defect is the weird lateral distortion in the print itself.

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Check the elasticity of the FEP film. Whether the fixing screws on both sides of the resin tank are tightened. Check whether the Z axis is loose.