Unfortunately, I am unable to reproduce the issue when testing this locally. Please see the attached screenshot. In my case, using the same values from your screenshot, the formula succeeds.
Can you confirm that all columns that are being used in this formula with numval() contains numerical values? If you simplify this formula and start with just one column, and slowly add each "part" or column one by one, can you confirm that it's this particular "hexane_flow_46fc218" column that's causing the formula to fail? Additionally, what version of DSS are you using?
If possible, would you also be able to share some sample data that exhibits this issue?