The first unit in the group or raid randomly disappears with no errors. I get "Interface action failed because of an AddOn" as a chat message at the same time.

It's a bug, and I pushed a fix for it on MovableFrames. It's a real bitch to manage header units like single, since they are re-positioned quite frequently onto the secure header. The fact that they are dynamically spawned doesn't really help either.

I ran a taint log. Seems the issue is with oUF_MovableFrames

5/12 04:06:54.538 An action was blocked in combat because of taint from oUF_Freebgrid - Raid_FreebgridUnitButton1:ClearAllPoints()
5/12 04:06:54.538 Interface\AddOns\oUF_Freebgrid\elements\movable.lua:161 Raid_FreebgridUnitButton1:SetPoint()
5/12 04:06:54.538 Interface\FrameXML\SecureTemplates.lua:804 configureChildren()
5/12 04:06:54.538 Interface\FrameXML\SecureTemplates.lua:1024 SecureGroupHeader_Update()
5/12 04:06:54.538 Interface\FrameXML\SecureTemplates.lua:610

What does the debugstack() say for the error?

Could you create a smaller example and some steps to reproduce it?

In a group, run oUF_MovableFrames with an oUF 1.4 layout. It now has every unit in the group handled individually with the movable plugin(in oUF 1.3, the entire header was one movable unit reguardless of the unit amount). I drag the first unit (unitbutton1) to the desired position(the rest stay anchored). Once I engage in combat the unitbutton1 disappears with the taint message.

Also, if the way movable handles header units with 1.4 is intended, I actually prefer it. If it wasn't for this bug