I'm interested to know why it is wrong, as the LOTG itself appears (to me at least) to be ambiguous.
Note that I'm a "Ref Dad" (my son is a young referee), rather than a qualified referee myself, so please go easy on me if I'm missing something obvious.
The text in the (2024-2025 English) LOTG says*:
"A flagpost, at least 1.5 m (5 ft) high, with a non-pointed top and a flag must be placed at each corner." [8. Flagposts]
Since "the lines are part of the area they enclose" [2. Field markings], the corner (of the field of play) is clearly at the intersection of the outside edge of the marked touch and goal lines.
However, the image in the LOTG [7. The corner area] shows the flag placed at the intersection of the centre of the marked touch and goal lines.
Similarly, when defining the corner arc, the image [2. Field markings] shows a line starting at the intersection of the centre of the marked touch and goal lines, but the text in the same section reads "Measurements are from the outside of the lines as the lines are part of the area they enclose".
Clearly the circumstances in which any of the above will matter are very few & far between, but it does seem ambiguous to me (and therefore perhaps not surprising that there is variation in practise).
Interestingly, I've only once noticed halfway flags being used at grassroots, and they had bizarrely been placed 'on the pitch', slightly inside the touchlines, when they are obviously supposed to be set back well away from the lines. They presented an interesting obstruction for wingers running up the line.
*NB LOTG quotes in italics, emphasis throughout is mine.
