According to the official docs, when sneak attack is deflected, it should do half damage.
However, this currently does not happen.
For most specials or regulars, there is a message. AS has “tried to hit but missed”, regulars have “glancing hit” etc. SA currently has no message. In addition, I’ve accumulted thousands of sneak attack damage logs and not a single one of them has done half damage or given any sort of deflect message.
Possibly related, there’s a phenomenon where a SA will simply disappear. No error message, no deflect message, nothing. If I had to conjecture, I’d say this is the deflected SA. SA is based closely on AS, so I suspect that the current deflect for SA is acting like AS (i.e., it misses), but without the error message.