The potassium in potassium bicarbonate is in the same form as the potassium metal is in after it has combusted.
When potassium metal undergoes combustion, it forms potassium oxide.
4K + O₂ → 2K₂O.
K₂O is an ionic compound. It consists of K⁺ and O²⁻ ions.
The K⁺ ion has a completed valence shell. It is a stable as it is going to get.
Potassium bicarbonate, KHCO₃, consists of K⁺ ions and HCO₃⁻ ions.
The K in KHCO₃ is in exactly the same form as the K in K₂O. It is as stable as it can get.
Potassium metal reacts violently with oxygen. Potassium ions are unreactive.