From @wardz:

There's currently a bug with sArena's savedvariables, causing them to duplicate and save gibberish (userdata) values. (Pastebin)
My sArena's savedvariables file is currently 397kb, 13727 lines long and keeps growing. Probably want to have this fixed first before releasing.
I also noticed there's a minor taint issue when trying to move(?) the arena frames in combat. For example, if an enemy rogue comes out of stealth while you're in combat, this causes the arena frame to be moved if it was not shown at arena start already, and thus causes UI taint. I think this taint only happens the first time the frames are moved/shown but not sure. Don't think this happened in Legion.

Just noticed it happens when dragging the trinket icon in testmode, if you drag the arena frame itself it doesn't bug out.
If you do /dump sArenaDB after dragging the trinket icon frame you'll see it tries to set data such as:
manatextright = { [0] = <userdata ArenaEnemyFrame3ManaBarTextRight[0]> }, and a bunch of other stuff. This also happens when sArena is the only addon enabled.

Saved Vars was fixed a while ago.
Taint fixed in sArena 3 rewrite.