Still experiencing this problem in various projects. Whenever a parameter is changed (specifically the length of the line and/or the Custom Marker Name), the grid line disappears from one or more stories, and the story settings cannot be restored. Cannot tell if this is Teamwork-related, as almost all projects within our office are now Teamworked, but here is my work-around:
Create a new file -- make sure the new file has same number of stories as the original. Trace the Grid Objects with lines in the original file and cut/paste into the new file. Create new Grid Objects with the desired settings (for instance, Show on Selected Stories...) and then cut/paste these into the original file. Seems silly, but it works.
MacBook Pro Apple M2 Max, 96 GB of RAM
AC26 US (5002) on Mac OS Ventura 13.5