Discord.js: DM channels only cache 1 message

[recapitalverb]

Please describe the problem you are having in as much detail as possible:
In a DM channel, there is only 1 message cached at any point in time.
This seems to be only occuring on master branch.

Include a reproducible code sample here, if possible:
Repeatedly evaluating this in a DM channel when there are messages being sent in it always return 1:

dmChannel.messages.size;

Further details:

• discord.js version: v12.0.0-dev (master branch)
• Node.js version: v12.3.1
• Operating system: Linux Mint 19.1 Tessa, xfce4
• Priority this issue should have – please be realistic and elaborate if possible:

• [x] I have also tested the issue on latest master, commit hash: 405bdb5b558eff478d0bde739cee1ee8e40c330b

[MrJacz]

AFAIK DM channels only exist in Client#channels when the bot itself creates the dm. so is this really a bug?
Considering you only get sent the channel_id and not the channel object.

[monbrey]

Just reproduced this myself trying to provide support for someone who was creating ReactionCollectors on multiple messages in a DMChannel - not possible since only the latest message was cached.

Confirmed with eval message.channel.messages.size always being 1