The error is around 0.1, which sounds like it is a floating point precision issue.
The Z value of a trace hit will vary based on the Z value of the start/end points of the trace. The larger the start/end Z range, the larger the variance of Hit.Z values.
Z value of trace hit varies even though object (Floor) and trace location are not changing
Start/End height of the trace does not affect the height at which the hit occurs.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-44797 in the post.