I've noticed this, not always. More like an occasional thing. I wonder if it is related to the amount of interaction with the watch, or certain types of interaction, can cause the phone to "work harder"? When I've noticed this, it seems to be on busy work days vs. weekend.
I haven’t noticed any significant change, but no iPhone X is going to hold a charge like it did when the battery was new. The max capacity on my iPhone X battery is currently 88%. I bought it about 14 months ago.
On the one hand the iPhone and watch sync data continuously, so that’s going to have some impact on the iPhone battery. On the other hand, as others have said, the watch cuts down on iPhone usage for some of us... which should help preserve the battery, but YMMV depending how much much your iPhone usage actually drops (if the amount is significant at all) while wearing the watch.
I think it would be unusual to actually notice the impact of the watch on the iPhone battery, so I would examine the individual app battery usage on your iPhone. There could be a third party app that is running amok. I would focus on the ones that have Watch apps installed. And if you haven’t already, give both devices a restart. If your Apple Watch is new, just be patient. There is a lot of data transfer and indexing that happens during the first few days and after some major watchOS updates.
My Xs has hit alot lower battery levels on a normal days use compared to when I didn't have my S4 watch..I actually saw low power mode for the first time the first week I had my watch. Usually don't go much below 50% battery level.
When I sleep with the watch and iPhone Xs (not charging) sitting on my desk the battery plot shows a 20-25% drop compared to 5-10% when I turn the watch off. It's busy chatting with the watch for something while I'm asleep.