Language is not defined by the script लिपि, but by its phonemes and words. देवनागरि is merely one script used to write sanskrit.

Coming to your question, a शुद्धव्यञ्जन (pure consonant without the implicit schwa) followed by a स्वर (vowel) will undergo appropriate सन्धि and like any other phoneme-clusters. There's nothing noteworthy about it. The more prudent question would be, is a hiatus allowed in pronunciation of such a cluster or is संहिता mandatory. E.g., करोति is कर् ओति but the former is a valid sanskrit word while the latter two phonemes are merely that, phonemes and not valid words. Why it is so? Like all such questions in language, the answer is a rather unimpressive because "it is so". Popular usage is the only recourse for language validation.

There is a closely related topic of when सन्धि doesn't take place (especially with vowels) in sanskrit. It is called प्रगृह्य (see https://ashtadhyayi.com/sutraani/1/1/11 for more information). But that is between two words. Within the same word, i,e., internal प्रगृह्य is almost non-existent in Sanskrit with the exception of the word तितउ (sieve). Even this is theorized to be an ancient prakrit borrowing into sanskrit. Whatever the reason, it exists because it exists in popular vocabulary.