I can confirm your observation: Actually, on my (virtual) test device running macOS 10.13.6 StitchBuddy was directly throwing an exception: The reason is an internal code change, introduced with the latest update, and not working well with this macOS version.
(Some technical background info for the interested: I switched some code from Objective C to Swift 5, and a required dynamic library "libswiftcore.dylib" isn't found when launching the app. I found this discussion in the Swift forum quite helpful.)
Although there seems to be a rough workaround, I decided to drop support for macOS 10.13, introduced 7 years ago and not maintained by Apple - even with security updates - since November, 2020. StitchBuddy 3.3.4 works well with macOS 10.14 and above. I'll update the app requirements shortly, sorry…
Edit (2024/08/02): StitchBuddy 3.3.4 was removed from the App Store as "last compatible version", so users of macOS 10.13 will be able to reinstall the latest working version 3.3.2…