It is possible that this is intended behavior:
The new transfer command intended to transfer players between servers is only able to be used by players, which, in my opinion, makes it very slightly pointless. So far, I have only tested it with command blocks and signs, but I am under the impression that it has a permission level higher than 2.
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Invalid, /transfer requires op level 3, command blocks and functions have access up to 2. (It's at the same level as /kick, /ban, etc.
Did you actually test signs, or assume they're unable to run the command because command blocks can't? Their click events are just player-run, with the player'spermission level and as the player as @s (just positioned at the sign).