The vowels of the last words of "Hui", "Lai", "Taiwan" and "Cup" are: ui ai ai ui.
In ancient rhyme, the whole tone of ui is uei, that is, the part used for rhyme is ei.
The prosperous Tang Dynasty of Du Fu's life is an important stage of the transition from classical poetry to modern poetry. Du Fu himself has also made great contributions to the metrical aspects of poetry, but these situations will inevitably appear in the poems in the transitional period, so this poem is one of Du Fu's few poems with loose rhyme.
I didn't notice it just now. I'm sorry