Yes, officially this is a documentation bug. From the Absent Fields doc:
Fields that are not always guaranteed to be set... are indicated in the per-type documentation. Any discrepency(sic) between observed behavior and the documentation should be reported on Stack Apps.
(Emphasis added and note that "discrepancy" is mispellerized. ;) )
notice obviously should have the may be absent flag.
I eyeballed all of the other properties of type question, as well as type answer and type user. I didn't spot any other obvious discrepancies with the may be absent flag.