Namaste everyone

The 14th mantra of the Isha Upanishad is as follows

सम्भूतिञ्च विनाशञ्च यस्तद्वेदोभ्यं सह ।
विनाशेन मृत्युं तीर्त्वा सम्भूत्याऽमृतमश्नुते ॥

In his commentary for this mantra Shankara Bhagavatpada says ‘सम्भूतिं च विनाशं च’ इत्यत्रावर्णलोपेन निर्देशो द्रष्टव्यः, प्रकृतिलयफलश्रुत्यनुरोधात् ॥

As far as I have understood Shankara is saying that there is a missing अ ( which should be added to सम्भूति to make it असम्भूति).

By adding an अ and making it असम्भूति then samuchchaya between asambhuti and vinasha can be undertaken (as previously explained)

My question is - how can the shruti mantra have a missing letter? On what basis is Shankara Bhagavatpada adding a अ to the mantra?

Obviously my understanding of the bhashya is incomplete and flawed.

Request the knowledgeable members to enlighten me.

Thank you