As noted in
other thread, this problem is not reproducible on any of our working and testing machines. The update providing the ArcsinhStretch module works on all supported platforms under normal working conditions, so if you are running version 1.8.5 of PixInsight and have installed all updates available, ArcsinhStretch should be available in your PixInsight installation. I suggest reinstalling the application, trying on a different machine, and checking your computer(s) for malwares, virus protection applications, or file management applications that can interfere with PixInsight. On Windows 10, some recent system updates have been causing many problems by erasing and damaging application files.
AFAIK, thousands of users are using the ArcsinhStretch process with version 1.8.5 of PixInsight. Unless you can provide a repeatable pattern to show that this problem happens consistently, these are machine-specific issues. Sorry for not being able to offer more useful help.