SymptomsMultiple subdomains are configured on the domain. If php as fastcgi is enabled on one subdomain, it is not possible to use php as apache module on another subdomains.
As soon as you change php mode to fastcgi on one of the subdomain, Plesk changes php mode on another subdomains also. Php mode as fastcgi will be set up for all subdomains of the domain.
ResolutionThis problem was discovered in Parallels Plesk Panel 9.x.
Fix for this issue was included into MicroUpdates for Parallels Plesk Panel versions 9.3. and 9.5.2.
For correcting the issue upgrade to version 9.3 or 9.5.2 and install all available MicroUpdates.
After that you should be able to set up different php mode on subdomains.