Neither above comment addresses the previous identified rear axle bump steer.
Thrust angle is an alignment problem. Rear toe on a straight axle is a bent axle. Both problems are not bump related.
Rear bump steer on OEM leaf spring setups is an intentional or acceptable chassis design compromise solution, and only corrected if unneeded or undesired by modifying the suspension, and not changed by simple alignment changes.
Disagree. Total toe on a straight axle can be correct AND individual toe can be incorrect. Yes, this phenomenon is usually called thrust angle but it comes down to incorrect rear toe angles. It really doesn't matter if it's a solid axle or IRS.
As for leaf springs causing bump steer. Yes they can. The spring oves the axle in an arc as ride height changes, effectively lengthening or shortening the wheel base. How severe this is depends on how level the front half of the spring is. Ideally the front half of the spring should be parallel to the ground at normal ride height. This minimizes side to side changes in rear toe.
