Can you attach your data? For that matter, we are still waiting to see your data for https://www.mathworks.com/matlabcentral/answers/404992-curve-fit-exactly-the-function-i-enter
Notice that all of the coefficients cross 0, and are all of the form (-V, V) . That tells you that the model the fit was about equally as good with negative coefficients as with positive coefficients.
When you see large negative coefficients being used within exp() then what it tells you is that the fit was trying to drive the associated term to 0. That can happen if you used a starting point that happened to be outside the basin of attraction of the good solutions: the fit starts to head towards the asymptopes of a bad solution.
When I look at your graph, it seems to me that you should be trying with x+1 or so rather than x-0.4
See https://www.mathworks.com/matlabcentral/answers/403639-exponential-fit-not-working#answer_322886 for how I dealt with this same curve shape for someone else recently.
