Me he de quejar de este | yerro (8)
Levantóse en su cóncavo | hueco (10)
Cual témpanos de | hielo endurecidos (11)
Tierno quejido que en el alma | hiere (11)
In none of these cases could there possibly be synalepha. Consequently by definition there can be no hiatus.
Hiatus most frequently occurs to avoid the greater cacophony which would arise from stress-shift under case 3 of synalepha:
Era la hora | en que acaso (8)
Lack of hiatus would here produce a stress-shift resulting in an unharmonious stressing of two successive syllables.
Reposaba, y tumba | era (8)
The same principle applies here as in the above, except that the effect would be even worse, because the stress shift would come under the rhythmic stress. (See below.)