I searched through the bug reports and did not encounter this one so I'm posting it.
I'm using 4.50.113 of the compiler. I have not tested this on the latest version and would appreciate it if someone with the latest version of Pelles C would confirm this.
There would appear to be a bug with the resource compiler related to the assignment of the Extended Style value to a custom control.
I am attaching a minimal project that reproduces this error on my system.
Viewing the resource script independently verifies that the value in question should be 0, however that is not the case when compiled.
Regards,
DMac