Email notification about new replies has broken avatar images

Pretty much the title. When I received an email notifying me that someone had replied to me on a thread here, the avatar images were ‘broken’. Example:

image

I assumed this might be an issue on the forum’s end, so I figured I should post about it here. When attempting to open the broken image in a new tab, it has a 404 page, like this:

image

If it’s not clear, the email service I use is gmail. I apologise if this issue is already known.

The googleusercontent URL is Google’s image proxy. If you go to the menu dropdown in Gmail and click “Show Original” then you can find the <img> tag in the email.
I just checked a recent email and the img tag is <img src="https://auth.spongepowered.org/avatar/simon816?size=45" title="simon816" width="45" height="45"> which works, but shows the broken image in GMail.
Not sure exactly why that is, some avatars were broken not too long ago so maybe google cached a broken image?

Hmm… opening https://auth.spongepowered.org/avatar/simon816?size=45 gets redirected to https://forums.spongepowered.org/letter_avatar_proxy/v2/letter/s/13f3d2/240.png?s=45. Could that potentially be why it breaks?

Otherwise, yeah. I’d assume Google cached a broken image as well.