Nixie tube multiplexer circuit problem
Your approach is based on the model to drive led matrix or 7 segment displays with all the cathodes combined. However a nixie is in principle a neon lamp. As soon as the voltage over the elements (single or in series) becomes high enough the digit(s) will be activated. This makes separating the tubes from each other absolute necessary.   

There is a more simple and straight forward way by driving each nixie with a 74141. Then the multiplexing is done at a TTL level. This way is a proven methode and should work without any problem.