I'm afraid I cannot look into this easily because the author saw fit to only distribute his game as a Windows-specific msi installer (which is obnoxious since the interpreter his game uses is multi-platform, obviously, since it works with iFrotz). I do not have a PC running Windows on which to try to reproduce the issue. If you can attach the actual .ulx or .gblorb game file here, I can look for general incompatibilities with desktop interpreters now, and look into Windows-specific issues later on a work PC when I have time. Or someone else might be able to help you in the meantime.
Frotz uses the standard glulx Quetzal save format. Save files made from manual 'save' commands (in other words, not autosave files) should be compatible across interpreters.
You may have to rename the file to have the extension the PC interpreter expects. Do you know if this issue is specific to Flexible Survival, or does it occur for you with other glulx games? What PC interpreters have you tried?