GuildMember#joinedTimestamp being null

[TAEMBO]

All properties mentioned here may be assumed to be under an instance of the GuildMember class unless otherwise mentioned to help with readability.

In one of my TS projects, joinedTimestamp as well as joinedAt have a type that is either a number or Date respectively, or null. Why is that? I've looked on the Discord server, and every answer I saw regarding the reasoning for them being possibly null is because it's a partial GuildMember. With that knowledge, shouldn't the nullified values be strictly put in PartialGuildMember instead? The d.js typings for a PartialGuildMember have joinedAt, joinedTimestamp, and pending set to where they may be null already, so why is this nullish typing behavior extending into the full GuildMember structure?

Furthermore, in the full GuildMember structure typings, the partial getter has an explicit type definition of false, which I believe most would infer means that the GuildMember that they're working with is a full GuildMember structure - joinedTimestamp and joinedAt not being null. With that being said, the definition of this getter is return this.joinedTimestamp === null;, which solidifies that if <GuildMember>.joinedTimestamp is null, then <GuildMember>.partial is true, and if it isn't null, then partial should be false. If the type definition for the partial getter in a full GuildMember structure is explicitly false, why is joinedTimestamp and joinedAt still being typed as if it can be null when the partial type definition says otherwise?

[Jiralite]

Receiving a PRESENCE_UPDATE event on an uncached guild member has no joined_at field (or guild member data as a whole), resulting in this being null.

[TAEMBO]

Ok, gotcha. With knowing that, that would mean that GuildMember#partial would be true in that instance as opposed to its type definition of false, yes? That seems to still leave typings that don't properly reflect their runtime values unless there's something else I'm missing. If this debacle is solely due to the presence update event, would it be better to label the new data as a union type of a GuildMember or PartialGuildMember instead of just GuildMember, and then remove the nullish option from these aforementioned properties from the full GuildMember typings?