I was not tenant and I was the owner. So I was responsible to pay the extra. And if tenant is in long teem rental, usually the tenant is responsible for all the expense. In the rental agreement, there should be a clause saying tenant pays monthly expense, except for the special assessment, like the cost to replace the front door or security camera or big paint job. In this case, owner, not the tenant pays the special assessment. The extra amount caused by some unit not paying, should not be part of the tenant responsibility. Other people cover the difference, it's really a loan to bridge the future payment. Eventually, that money will come back to the owner. So it's not fair for tenant to pay. On the surface, it's the expense, but it's owner responsibility to cover it. The tenant and owner should discuss about this.
If the lease says, tenant pays all expense except special assessment, then it's important for tenant and owner to sit down to talk about this.