Yashkur has the form of an imperfect verb, but the book treats it here as a proper name.
قَابَلْتُ يَشْكُرَ. — I met a person named Yashkur.
قَابَلْتُ: past verb, subject تُ.
يَشْكُرَ: accusative object with fatḥa; a diptote proper name.
The initial yāʾ alone is not the cause; the pattern and proper-name status must be considered.
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Diptote Causes — Verb Patterns and Their Conditions
Yashkur has the form of an imperfect verb, but the book treats it here as a proper name.
قَابَلْتُ يَشْكُرَ. — I met a person named Yashkur.
قَابَلْتُ: past verb, subject تُ.
يَشْكُرَ: accusative object with fatḥa; a diptote proper name.
The initial yāʾ alone is not the cause; the pattern and proper-name status must be considered.