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Taking chunks?    Unless you converted the entire aggregate amount of all Traditional IRA accounts and the year end balance of all accounts was zero, then that is not a backdoor Roth and only  a part of 2018 should have been offset.

It sounds like you failed to enter the 2018 December 31 value of ALL existing Traditional IRA accounts and the total nondeductible basis was improperly applied when it should have been pro-rated between the 2018 distribution/conversion and the remaining IRA balance.   Now in 2019 there is no longer any carried forward basis to apply.

To verify look at the 2018 8606 form line 14.  If zero or blank then that is probably the problem.  Then look at the 2018 8606  form line 6 (or if there is an * [asterisk] next to line 15, then look on the "Taxable IRA Distribution  Worksheet" line 4).    If blank or zero then that is where the year end balance should be and you need to amend 2018 to correct it and then use  remaining 2018 new 8606 line 14 value to input into the 1099-R interview for 2019.   This should mean that you will owe affiliation 2018 tax because it was under-reported originally.