There are two variations of spellings for Salāh in all the copies of the Mūs'haf (even printed in different parts of the world). Both the variations in spellings are NOT used interchangeably i.e., whenever wished written with a Wao and whenever wished written with Alif without a Wao. It is not like that. It is seen, when it appears as a proper noun (المَعْرِفَةُ) with (ال) as a prefix then it is written with a (و) and (ألف خنجرية) over it (وٰ); - (ٱلصَّلَوٰةِ). But when it comes with possessive pronouns (اَلضَّمَائِر الْمُتَّصِلَة) then it comes with Laam Alif (لا) e.g., (صَلَاتِي), (صَلَاتَهُ), (صَلَاتِهِمْ), etc. However, there is one exception in Sūrah Hūd, Āyāh 87 where it appeared as a possessive pronoun but with a (و) and (ألف خنجرية) over it. Here below is the Āyāh of Sūrah Hūd.
قَالُوْا يٰشُعَيْبُ أَصَلٰوتُكَ تَاْمُرُكَ اَنْ نَّتْرُكَ مَا يَعْبُدُ اٰبَاۗؤُنَآ
I assume that since (أَصَلٰوتُكَ) is prefixed with an “Interrogative Hamza Alif” (الف همزة للاستفهام), which sounds like the prefix (ال) of a proper noun, therefore due to resemblance in utterance (sound) its spellings are treated as the Salāh in the form of a proper noun!!!!!!!!! The other Āyāāt where it appeared as a possessive noun and is written with Laam Alif (لا) are as follows:
قُلْ إِنَّ صَلَاتِي وَنُسُكِي وَمَحْيَايَ وَمَمَاتِي لِلَّهِ رَبِّ الْعَالَمِينَ
وَمَا كَانَ صَلَاتُهُمْ عِنْدَ الْبَيْتِ إِلَّا مُكَاءً وَتَصْدِيَةً
كُلٌّ قَدْ عَلِمَ صَلَاتَهُ وَتَسْبِيحَهُ
الَّذِينَ هُمْ عَلَىٰ صَلَاتِهِمْ دَائِمُونَ
وَالَّذِينَ هُمْ عَلَىٰ صَلَاتِهِمْ يُحَافِظُونَ
The reason for variations in such spellings is in accordance with the rules of (الإعلال والإبدال). But the question here is not whether it is permitted in Rasm al-Uthmani or not rather the question is why in the Mushaf (where the same writing script Rasm al-Uthmani is used throughout), do we see such variations. It is not by choice to write the desired spellings but the reason is governed by a certain rule which we want to know clearly.