The final letter of حُبْلَى is an alif, which cannot carry the audible case vowels. Thus its case marker is estimated.
Estimated case does not mean the word is indeclinable. حُبْلَى is an inflected diptote; its case markers cannot be displayed on the final alif.
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Causes of Diptote Status — Femininity and Its Conditions
The final letter of حُبْلَى is an alif, which cannot carry the audible case vowels. Thus its case marker is estimated.
Estimated case does not mean the word is indeclinable. حُبْلَى is an inflected diptote; its case markers cannot be displayed on the final alif.