Translucent objects do not write depth and cannot participate in DOF. They will get the DOF of the nearest opaque object behind them.
The cine camera focus does not blur transparent materials.
Found In 4.18 CL# 3832480, 4.19 CL# 4033788, 4.20 Preview 5 CL# 4173640, 4.21 CL# 4195822
The translucent cube will always been sharp and in focus while the cube with the basic shape material will come in and out of focus based on the cameras distance
Both cubes will blur and sharpen properly based on the cameras focus distance
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-61596 in the post.