Hi, this is likely related to how popup window position is computed.
(see How to adjust the placement of the popup for editcomptext plugin for in depth information)
First, in my version of TW, I cannot trigger EditCompText from a field, so I am surprised (in a good way).
Second, as said in the linked post, EditCompText is currently developped to work from the body. Using it in a Field is thus likely to get unusual effect. Sorry.
I might find the time to code a newer, more robust EditComp… I’m thinking about it.