When the CharacterMovementComponent makes the transition from falling to walking, it retains the forward velocity it had while falling. When the character lands on the moving object, its new velocity is relative so it should be 0, however since it still has the falling forward velocity the character skids forward, making the character move faster than the platform they are standing on for several frames.
This was reported and tested in 4.19.2 (CL-4033788). This was reproduced in 4.20.3 (CL-4369336) and Main 4.21 (CL-4383127)
Results: After jumping, the character will land back onto the moving platform and slide forward.
Expected: After jumping, the character should land and have 0 velocity or slide in the correct direction so it appears to be gaining velocity, not teleporting forward.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-64347 in the post.