So, I think I f***ed up my 2019 returns.

Yesterday we received a notice of proposed changes to our 2019 return which results in an owed balance of just over $11K. This is a result of missing 1099-B forms on my end. Forms that I cannot find (but know exist). The fact that these are missing is my own fault - they're easily accessed on my employers intranet, but as a former employee I no longer have access to that and as a former employee I no longer seem to be highly prioritized by the company's HR department (at this rate it'll take several weeks for me to get these).

The 1099-Bs come as a result of RSU dividends of approximately $36,000. In the proposed changes sent by the IRS, they propose changing my taxable income (line 11b) from ~91K to ~127K for the year. My understanding is that:

• RSU income should be included in Box 1 of my W2, which for 20019 reads ~91K (this makes sense to me; I had a base salary of 55K plus the RSUs equals 91K)
• shares are automatically sold to cover taxes on RSUs at the time of vesting (in this case, also the time of my withdrawal)

In the proposed changes in the IRS letter my income has been both adjusted upward and treated as if it has been untaxed. I know (think) that I am going to need to file an amended return for 2019 that contains the missing 1099 forms. The big bold "Proposed amount due: $11,110" has me a bit of a panic though. Are either of my assumptions above incorrect and is this something that will likely be easy to fix with an amended return?