These are areas where the parameters result in numerical problems and results should be taken with a grain of salt as they may be numerically imprecise. That’s why you get the warning. Inversion of a near-singular matrix is required in those steps.
Regarding your last statement: it’s not true that 0 is the highest value of a log-likelihood as we are talking about pdfs of continuous distributions. See [Computing Log Likelihood and Optimization)
Also, when you say high likelihood values, are we talking about the actual likelihood or minus the likelihood. Because ideally those problems result in low likelihoods and the draws are rejected.