When I initially performed PCA on my data, I noticed samples segregated mostly by sex (with the effect of interest being drug treatment). As such, I changed my design formula from just design = ~age to design = ~ sex + drugtx. This did not change the PCA plot relative to my original design formula, with samples still clustering by sex (although the significant DEGs did change). Is this to be expected? Why does the PCA plot not change when design formula is altered in this case?