The unofficial servers have always had shield wrong, imo.
Every one I've been on (as a career wiz from when mir2 was invented) it lasts seemingly forever. I'd have wished for a shield that behaved this way, it never did.
I also recall shield having, if you like, an imaginary HP, or hit count. So as long as your shield would last whilst not being hit, if you were being hit, the HP had a chance of running out and hence the time shorter.
Only thing I can think of is that it changed officially from 1.9 onwards, if the original sources were used from that point.