This is an old bug Origin 8.

Workaround was to use 2D graphs toolbar button to change plot type.

Steps to reproduce:

1. Create a graph with line+symbol plot.

2. Set the symbol to use Single Alphabetic characters

3. open Plot Details to change plot type from Line+Symbol to Line

4. Click OK and save project.

5. Reopen the project.

==> The plot is still line+symbol.