keycode string of arrow keys don't work in command-line expression

I found that when typing in the command line, using ^R= followed by a string of keycode "\<C-Left>" or "\<C-Right>" is not working as expected. Instead of bringing the cursor to the beginning/end of the current word, they resulted in strings like this:
:MyCommand<80>ýU
after typing :MyCommand followed by ^R= and then "\<C-Right>"

Keycodes other than the arrow keys, like "\<C-w>" and "\<C-e>" works properly in the command line expression.

I like to use these keycodes in some custom key-mapping, is there any way to work around this? The version is 8.0.540

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