Baek and Zheng’s Threshold Cryptosystem - limit on threshold?
The problem (thanks for the prompt Ido) was the use of PBC's element_mul_si instead of element_mul. This caused the polynomial calculation to loose precision(by rolling over I think) on larger values of t.

I now use element_mul during polynomial calculation in this commit and it works perfectly. Python Charm does not use element_mul_si hence the issue does not show up in the HoneyBadger implementation of TCG.