I have several Windows servers that the host=$decideOnStartup, but other Windows events correctly provide the Windows host name. Any ideas why and how to correct this?
Some modular inputs including streamfwd do not respect $decideOnStartup variable.
I think you deleted $SPLUNK_HOME\etc\system\local\inputs.conf file or delete host definition. Normally inputs gets the %COMPUTERNAME% environment variable to use as host field, but stream app.
As a resolution you should create inputs.conf on the forwarders that runs Stream add-on like below sample;
host = system_hostname