It is because of the hidden sound 'ㅂ' 여덟 is pronounced as [여덜] standalone or before consonant. But, when it is followed by a vowel, it is pronounced as 여덜ㅂ For example, 여덟이다 => [여덜비다] 여덟에 => [여덜베] BTW, most young generation native Koreans tent to pronounce 여덟이다 as [여덜이다] nowadays. The hidden consonant sound also occurs in French like 'restaurant, art'

What does the 'n' do in 'Autumn', 'damn', etc? What does the 'b' do in 'thumb', 'dumb', etc? Korean has much less of these exceptions than English. Just memorize them. Sometimes it's much more productive that way.

It needs time to used to.