New congruences on multiple harmonic sums and Bernoulli numbers

Let ${\mathcal{P}_{n}}$ denote the set of positive integers which are prime to $n$. Let $B_{n}$ be the $n$-th Bernoulli number. For any prime $p\ge 11$ and integer $r\ge 2$, we prove that $$ \sum\limits_{\begin{smallmatrix}{{l}_{1}}+{{l}_{2}}+\cdots+{{l}_{6}}={{p}^{r}}\\{{l}_{1}},\dots,{{l}_{6}}\in{\mathcal{P}_{p}} \end{smallmatrix}}{\frac{1}{{{l}_{1}}{{l}_{2}}{{l}_{3}}{{l}_{4}}{{l}_{5}}{l}_{6}}}\equiv-\frac{{5!}}{18}p^{r-1}B_{p-3}^{2}\pmod{{{p}^{r}}}. $$ This extends a family of curious congruences. We also obtain other interesting congruences involving multiple harmonic sums and Bernoulli numbers.