paste works once, register then garbled

ViM 7.3.46 on Windows XP
big version with GUI

1. copy block of text to register a with "ay6

2. record macro sequence
    /\t21
    "ap;
   
3. result first time: cursor moves to first line containing [tab]21, content of register a pasted below line

4. result next time: gibberish string pasted in - see screenshot if the line below doesn't show solid blocks

█K8\t█KE█KD█kb^M"ap^[^[

What's going on? why are the keystrokes being pasted in instead of interpreted?

Thanks in advance for helping me sort this out – life's too short to do hundreds of copy/paste manœuvres by hand :-}

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