analog multiplexer CD4051BE
I have just repeated your test with 3 random different 4051 ICs.

I just used a DVM to measure the voltage on the multiplexed pin - pin 3.

In each case, the voltage on pin 3 was the same to 3 decimal places and was the same as the voltage on the potential divider on the input.

I presume the problem must be in the way you are reading the analogue input.  Have you tested the voltage on pin 3 with a DVM to verify your results?