Sometimes my raid frames don't fully load, and can only see a couple of the raid members. I have to /reload to fix this.

This is an issue with the game client i think. They will populate after a short while automatically or you can as you said reload the ui.

I have confirmed this is an issue with the client itself. Other addons have the same issue. This might be an old bug that was brought back since classic is based on an older client. The bug was discussed here: https://www.wowinterface.com/forums/showthread.php?t=49875?pda=1

I have reported this on the current PTR. Maybe blizzard will fix it.

What's stopping the addon from periodically re-checking raid roster? Or doing so a second after someone joins the raid?

The groups are created by blizzard code. All unitframes just put their layout on top.

I made somewhat of a workaround but i am gonna leave this open until blizzard officially fixes it.