For anyone who may stumble upon this post in the future, the change in behavior from 1.6.2 to 1.7.3 was due to a rounding error. The rounding error caused a covariance matrix, cov_mat, to become asymmetric: cov_mat[1,2] - cov_mat[2,1] = 1e-16. This triggered an isposdef(cov_mat) if-else statement, which revealed a bug in my code.
For whatever reason, the rounding error only occurred during compiled mode and not in interpreted mode. This made the issue especially hard to debug. But ultimately it was an issue in my code which caused the instability.