I tried to see what's going on with your forms, but it's taking too long, so I decided to test it with a new form, which seems to work. My guess is maybe your form got corrupted. Just my 2 cents...
Instead of assuming the previous record is only 1 value before the current one, you could search for the highest autonumber value that is less than the current one.
Glad to hear you got it working, but using -1 in your expression may not be reliable. Potentially, you could end up having gaps in your Autonumber fields and -1 will end up failing. Just a thought…
Can you show us your CF expression? Are you using an Autonumber field as the primary key? Or, so you have a date field to help determine which record is the “previous” record?
Still only using my phone, so can’t see what Arnel provided, but I have a feeling it would include a similar function if it was needed. If you’re still curious to see what my function looks like, you can go to this link.
SimpleCSV()