In this caze z=99/100 is not so close to 1 when the precision is 50 to explain the bug. Simply adding 10000 terms of the series provides 40 digits.

As a workaround:

> convert(hypergeom([2/3, 2/3, 2/3], [-1/3, 4/3], 99/100),MeijerG):
> evalf[50](%);

   -138.99623131713330917549791383444227380249824260751

   +8.3550383462461792073697301492096375870611508951167*10^(-47)*I