CD4067BE Multiplexer input always observed at all outputs
open-circuit voltage of the current source is 100V


You are operating your CD4067B outside the allowed limits, and you might have permanently damaged it. The absolute maximum voltage allowed on any input is \$V_{DD} + 0.5V\$ (you have stated that you are using \$V_{DD} = 15V\$).

From the TI CD4067B datasheet:



Also you said:


  I tried changing one of the selection inputs to (5V)


When you have \$V_{DD} = 15V\$, then \$5V\$ is not a logic 1! Again, from the datasheet, with \$V_{DD} = 15V\$ then \$V_{IH\space Minimum} = 11V\$