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You certainly aren't alone there, don't worry, it's just that they don't really tick the box for indicating a rearrangement of the letters, that's all. They do appear in lists of 'acceptable' anagram indicators both online and in print, even Chambers Crossword Guide, but more experienced setters tend to avoid them. Here is a lovely example of the correct use of 'could be' in conjunction with an anagram indicator, in a prize winning &lit Azed Competition clue:
Ham, a slice of Granary, plus a couple of onions (pickled)? Could be ——’s (9)