I am going to page @Rick19744 for this.

However, it appears as if there would be a contribution to capital and then a special allocation to Member A.

Some initial thoughts: 

• The overriding mechanics of partnership allocations is that they must either be in accordance with the very complicated substantial economic effect regulations or based on the partner's interest in the partnership (PIP).
• There are some missing pieces of information
  • What is the intent behind Member A covering the expenses?
  • Is this a loan?  If 'yes",  this should be put into writing and a fair interest rate charged
  • Is this a capital contribution?
    • If a capital contribution, will Member A receive an increase in ownership?
• Having a better understanding of the above will help in providing some guidance on how to complete your tax return.
•  

Member A merely covered the expense until 2021 income could cover it.  So I suppose it's a "capital contribution."  No change in PIP or ownership (it was not considered significant; Member B could just as well have paid the expense).

The LLC checks "Yes" on Schedule B Question 4 so my understanding is that capital accounts don't need to be listed on K-1.  So can the LLC just list 0 across the board on K-1s Box III, and Member A can deduct that as a loss on his Schedule C?

I see another issue.  it's a question of whether member B has $5,000 of basis to take his share of the loss. if one argues that A gifted 50% of the $10K (as a capital contribution) to B. then B has no obligation to repay A. if he does, then one can argue no gift was made and B has no real basis.  this is a situation that should be discussed with a tax pro. Certainly, others can have different viewpoints. 

@Mike9241 they should be good to go following the provided guidance.