Update Jul. 28 2018:
OnePlus has released a new version of Oxygen OS 5.0.4, in which this issue has been fixed. This new version is available as update via Oxygen Updater and OTA.
OnePlus recently released Oxygen OS 5.0.4 for OnePlus 3. According to its changelog, it should feature the July 2018 security patches.
However, when you look in the settings app or in the Device Info section of Oxygen Updater, you'll see a security patch date of June 2018.
According to OnePlus, this difference is caused by a typo. They've assured that the security patches of July 2018 are in fact present in version 5.0.4.
Currently, no security researchers have confirmed if this is actually the case. It's also not known if a revised version of Oxygen OS 5.0.4 will fix this issue, or that you'll have to wait for the next update to get the correct security patch date.
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