FYI i can copy an item link in DEVONthink 3.3 ToGo to the clipboard then paste into an OmniFocus 3.12.1 new item title but not b to the Note for that item. Note is where it should go. Even the paste label does not show on a press into the Note field.
I no longer use OmniFocus so do not know when this started.
All works fine when pasting into Things.
So i think your hunch that it is an OmniFocus issue is on target.
Something seems to be wrong in OF, as I can’t paste item links as well.
As a workaround, you could use the built-in OmniFocus share option of DTTG (that uses their URL scheme I think) or try and built a shortcut that adds the UUID link to an action.
Extra information: the URL link from Things also cannot be pasted into OF, so that excludes DT as the problem.
It seems that a (weird) workaround exists by pasting the UUID link into another app, copy the UUID link from that app again and then paste it into OF.
You could make that somewhat easier by creating a shortcut:
add the Get Clipboard action
add the Text action
add a magic variable to the Text action and choose the output of Get Clipboard
add the Copy to Clipboard action (which should take the Text output as input)
You might want to place the shortcut on the home screen for convenience, If you then copy the DTTG item link, run the shortcut and open OmniFocus, you can copy the DTTG item link again.