I see how that works, but I really believe it's long; well, it's normally long.
It is short, while the -i of mihi is long:
Quandŏquĭ|dem for|tūna mi|hī tē|t(ē) abstulit | ipsum.
Quandō, in this compound or on its own, can have a short -o. Some more examples:
MEN. Līber(em) ego tē? | MESS. Vērum, quandŏqui|d(em), ere, tē servā|vī. MEN. Quid est? (Plautus, trochaic septenarius)
Quandŏquĭdem penetrant in eum contāgia morbī. (Lucretius, dactylic hexameter)
Vēr(um) age, quandŏquĭdem fātīs urgētur acerbīs. (Vergil, dactylic hexameter)
Quandŏquĭd(em) et il|lī peccant, quōs | minimē putēs, (Phaedrus, iambic senarius)
Dīc mihi crās istud, Postume, quandŏ vĕnit? (Martial, dactylic pentameter)
Quandŏ, prō Tītān, | ubi quō sub axe (Seneca the Younger, Horatian Sapphic)
Aliquandŏ siti|ēns ūnus dē | custōdibus (Phaedrus, iambic senarius)
In fact, I wonder whether quandōquidem with a long -ō- is attested at all... Maybe there's a line of Plautus or two with it. You can compare it with "nesciōquis", which is generally nesciŏquis with a short -ŏ- as well. (I similarly wonder whether nesciōquis is attested...)