Today it is always pronounced as 여덜 even when followed by a vowel-initial particle. 여덟이 is 여더리, not 여덜비.
여덟 • (yeodeol)
First attested in the Seokbo sangjeol (釋譜詳節 / 석보상절), 1447, as 여듧 (Yale: yetulp).