I'm not convinced this is not a data entry error on the user's part. This can easily happen because there are many things the program does not (in my opinion) provide enough clarity on. Then there's other things the program provides more than enough clarity on, but the user for whatever reason, does not read the small print.
On thing you can do is figure it manually yourself, to see where the user (most likely) is messing up. But you have to figure it the way the IRS says to figure it. (Which is what the program does.) Take a look at IRS Publication 946 at https://www.irs.gov/pub/irs-prior/p946--2019.pdf On page 38 is the worksheet to use. For residential rental real estate you'll use table A-6 on page 72.
Things that make this happen.
1) If the business use is less than 100%, then the cost basis of the business use portion will be less than the overall cost basis. So even if you placed the asset in service on 10/15/2021, the business use percentage as of that date is ONE HUNDRED PERCENT unless you really do have personal use after converting the property to a rental. What you used it for before converting it to a rental does not count for anything.
2) If you declare more than ZERO days of personal use, unless you actually have days of personal use after you converted it to a rental. Take note that personal use days should be zero, as it's only asking for days of personal use "AFTER" you converted the property to a rental. What you used it for before the in service date *does not count* for anything and therefore should not be included in any "Personal use" days you may have after converting it to a rental.
3. If you claimed a SEC179 deduction on any assets aside from the property itself, that too will reduce the overall cost basis total of all assets, by the amount of SEC179 claimed.
4. If you claimed the Special Depreciation Allowance on any assets, that too will reduce the overall cost basis total of all assets by the amount of SDA claimed.
