It is not wasted.
Feeblemind is a spell effect. It sets the ability scores to 1 while it is in effect. However, the creature's original scores are not erased and the ASI is applied to these original scores. Take note that under "Specific beats General" the original ability scores are too general, under the spell's specific directions. You have two ability scores for INT and CHA during the effect of feeblemind, but the more specific wins. It is like a Barbarian wearing armor. He has two sources of AC, but can only choose one. In this case, you can't choose but there is a precedent for two values for one trait. When the spell ends, the original scores (modified or not by ASI) return.
On a failed save, the creature’s Intelligence and Charisma scores become 1.
This effect surpasses the ASI. Do not apply the ASI to the score of 1.
At the end of every 30 days, the creature can repeat its saving throw against this spell. If it succeeds on its saving throw, the spell ends.
The spell can also be ended by greater restoration, heal, or wish.
When the spell ends, the effects cease to be. The INT and CHA scores return to their original values. If any ASI was applied to them, they take effect immediately.
When you reach 4th level, and again at 8th, 12th, 16th, and 19th level, you can increase one ability score of your choice by 2, or you can increase two ability scores of your choice by 1. As normal, you can't increase an ability score above 20 using this feature.
However, your DM may rule you cannot apply the ASI to either score during the effects of the feeblemind. The wording "can" implies player agency. This DM sees no problem in allowing a player to postpone applying the ASI to a later date (they are losing the improvement during that time anyway).