@acer Thank you for letting us know that the bug is already reported.

@Preben Alsholm Yes, it seems evalf is safe, even for large Digits. I agree using plot as a play model to evaluate the functions is not good. Although I think Maple should revise the command so that outputs like those are not produced. It is worrying, at least. Thank you for all your responses.

@dharr Our actual program is too complicated (it combines calculations with Maple and Matlab). Since there seems to be something strange in the output we started to analyze piece by piece. We thought that the evaluation routine  might be wrong in Maple 2023 when Digits is large, since the plot command must use somehow the evaluation routine to generate the plots, I guess. Apparently evalf does not show this problem. We will look somewhere else. 

@Preben Alsholm Thank you for your answer. Yes, for plots your approach seems to solve the problem. If you look at the actual data Maple is plotting (without adaptive=true), the values are negative. So it has evaluated the function incorrectly. That was my point. I used the plot command as a toy model. My problem is the way Maple evaluates numerically expressions when Digits is large. Because I seem to get non-sense in my data when Digits=40.

@tomleslie Thank you very much for your answer. Yes, using add will do the job. This was a toy model of my real computation. But I do not need a closed formula for the summation. So your proposal will work for me.

Your approach will do either. Now I have several choices to solve the problem. Thank you very much.

Thank you very much. If I decide to use alias (which seems to be risky), both your suggestions will do.

The reason I chose alias was I had to solve vector partial differenctial equations. What I showed was the easiest case where I had the same problem I was finding when working on the real system (more functions and more variables). Best regards.

Thank you very much for your answer. Now it is clear to me what alias was doing. Regards.