“Does disabled face id affect front camera portrait mode”

I cannot say this for sure, because I have never tried, but my inkling is that it should still work. Although the more I think about it, the more I am uncertain. It probably depends on why face ID is disabled.

The front facing camera actually has very little to do with Face ID, believe it or not. Most of Face ID's functionality comes from infrared based components (which is how it functions even in low light). But portrait mode may make use of some of the IR components.