Here is a small package to illustrate the issue:

https://github.com/mllg/r6pkg

Everything works out fine while loading the package interactively (using devtools' load_all()):

load_all()
print(bar)
<foo>
  Public:
    clone: function (deep = FALSE)
    x: 10
    x_squared: active binding

The active binding is lost after installing and loading the package in a fresh R session via library():

library(r6pkg)
print(bar)
<foo>
  Public:
    clone: function (deep = FALSE)
    x: 10
    x_squared: 100

Looks like the binding gets evaluated during R CMD build and the returned value is stored instead.