Some time ago, Rubick's Tempest Double was able to steal spells normally. However, this ability was taken away from it, as an attempt to avoid a bunch of bugs the double had with channeling spells (some channeling spells would last permanently if the double expired during them).

This change, however, did not do anything but unecessarily nerf Rubick. The channeling bugs were still abusable without the help of the double, so they had to be actually fixed, which happened. So now what we have is:

* Rubick double unable to use spell steal to avoid the channeling spells bug
* Channeling spells bugs ended up getting fixed, since they were abusable either way
* Double is now unable to use spell steal for no reason anymore

His double should be able to steal spells again, because not only did the change/nerf fail to achieve its point, it also got irrelevant since the reasons were fixed.

There still are some minor bugs the double has with some stolen spells, but these bugs are also present without the double (you don't need the double to repro these bugs).