if I had personal expenses in 2018, and I filed a reimbursement in 2020, would that be an issue?
I wouldn't know that for a fact. But requesting reimbursement for expenses from two years ago would definitely raise flags. I have no doubt of that.
But there is one thing you need to understand here, that I've not seen clarified in this thread yet.
If the S-Corp does *NOT* have an "accountable reimbursement plan", then whatever you get reimbursed is included in box 1 of your W-2 and *is* taxable income to you.
If the S-Corp *DOES* have an accountable reimbursement plan, then what's reimbursed to you is not taxable income and is not included in box 1 of your W-2.
For information on what constitutes an "accountable plan", see IRS publication 463 and you want to read Chapter 6 - Reimbursements. https://www.irs.gov/pub/irs-pdf/p463.pdf
