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I was taught that when a "weak" vowel (i, u) combines with a "strong" vowel (a, e, o) you pronounce them as a single syllable diphthong. But an online dictionary says that the verb criar is pronounced with two syllables. At least in the pronunciation popup they spell it "kree-ahr".
Is there another rule governing the pronunciation of "criar" that says it should be considered two syllables?
Yes, criar is pronounced (by everybody I know, at least) with two syllables, /kri.ar/. This is a violation of the rule that says that a so-called "weak"¹ vowel (/i/ or /u/) always forms a diphthong with a following "strong" vowel.² It's not the only one: it appears (for some people, not all) in several other verb infinitives ending in -iar or -uar such as enviar and actuar. (This answer and part of this answer are also relevant to your question.) It also happens in many other words that are not verb forms, such as adjectives (dual, fiable) and nouns (cliente, cruento).
The hiatus (i.e. the breaking of the expected diphthong) is unpredictable, but it tends to appear, according to studies, due to two main kinds of factors: morphological and phonological.
The morphological factors can be summarized like this: a diphthong will be broken in a word if it gets broken in words with the same root. So for example, since the present tense of the verb enviar are envío, envías, envía, etc. with a hiatus (shown by the accented í), the infinitive enviar itself will be influenced and will be pronounced with a hiatus, en-vi-ar.³ There's more to it than that but that's the idea.
The phonological factors include the presence of a liquid (r or l) in the onset of the syllable, i.e. before the main vowel. In criar we have a stop consonant /k/ (c), plus a liquid /r/, plus a vowel /i/ that should phonetically become a glide or semivowel [j] (like the English y in yet); in many words this pileup seems to be too much so the speakers spontaneously break the syllable in two to preserve a simpler structure: /kri/ + /ar/. This may be also what happens with cliente and cruento. (Again, there are more factors in play.)
Spanish marks basically everything in orthography, but it doesn't show this irregular hiatus in any way.
¹ "Weak" vowels are technically close vowels, while "strong" vowels are open (/a/) or mid (/e/ and /o/) vowels.
² Actually any "weak" vowel is apt to form a diphthong with another vowel, of either type; iu and ui are possible.
³ The subset of the -ar class of verbs with an infinitive that ends in -iar or -uar is thus divided into two subclasses: those where the diphthong is kept (like anunciar, cambiar, averiguar, menguar), and those where the diphthong is broken in certain persons and tenses (like criar, enviar, adecuar, actuar). See the RAE's conjugation paradigms.
