Tom, I think you are right. I am working on my 2020 tax as a trader and looked into the same problem. I used Charles Schwab and in 1099B the "washed sale disallowed" amount did add to the cost of the replacement shares. One can check this by comparing each transaction where washed sale disallowed appears with a number in 1099B with trade monthly statement the broker provided. I am not sure about other brokers but I believe they should run on similar algorithm. Anyway, worth to test using the comparison I mentioned. Simply excluding the washed sale disallowed amount without understanding is not a proper way. Hope this can help to clarify...
