Product and version
Filemaker Pro Advanced 220.127.116.110 x32 (bug was also present in v2)
OS and version
Windows 10 Pro Version 1809 17763.316 x64
This is a low level issue, but I had to report it , it did the same with all layout that has an underscore in the name
When editing a script,
script step "New window", you select a layout in the list, if the layout's name have an underscore (or multiple) the 1st one is remove when you select it.
But will display ok in the Script Workspace
In the script step "Go To Related Record" that as a similar option page and you setup a layout, it will keep both underscores.
How to replicate
Just name you layout with an underscore.