I am still trying to realize Barnabas's need for medical treatment, in my mind, and the only conclusion that I can come up with is that somehow his 1840 self and 1970 self melded together when he returned to 1971. So that the uncursed meshed with the cursed, forming a creature with vampire dna that still needed a cure. Sounds corny, I know, but it is the only way I can reconcile this discrepancy.
There has always been some debate in fandom over whether or not Barnabas was still a vampire after he returned to 1971. Some fans feel that because Angelique removed his curse in 1840/41, he was no longer a vampire in 1971. However, because Barn's curse in 1971 was caused by Jeb and not Ang, other fans argue that he IS still a vampire. And adding credence to the latter belief, Sam Hall also considered Barn to still be a vampire when he wrote his TV Guide article.
Truthfully, having had Angelique remove the curse in 1840/41 causes huge problems, the least of which is whether or not Barn is still a vampire in the present. The biggest problem is if Barn was cured back then and never rechained in his coffin, how the hell do the people in the present even know who he is because Willie would have never been able to unchain him in 1967!
But as we know only too well, the DS writers never considered that sort of stuff and often left it to the audience to figure it out for themselves, which is no doubt why we're still debating it.
And basically the only way to fathom it all is to theorize that somehow the I Ching or the stairway through time or some combination of both took care of getting Barn's 1840/41 self recursed and rechained in his coffin as Barn travelled to 1971. But how that actually took place is anyone's guess because, well, the writers weren't about to tell us - even if the show had continued past April 1971.