I figured it out. It is kind of a bug, but also my mistake.

The field the formula is referencing contains an IF statement. When that IF statement is true, there's a formula. When the IF statement is false, I had put apostrophe apostrophe ('') in an attempt to leave those cells blank. Back to the field that's throwing all errors, it was interpreting the other field as text, so throwing all the errors. But as for why the addition worked and not the multiplication, that's weird.

Anyway, the solution for that other field was just to have nothing for the false IF statement, like this:

IF({Plantings}>1,{Units} * {Weeks})