Checked my release. Funnily enough, when I uninstall iPhone version (as I mentioned, bundled but standalone) the notifications on the Watch start to arrive precisely on time.

Same issue here, WatchOS 8.3. What's even funnier, same code runs fine on one of my apps (standalone app), but not the other (well, also standalone but bundled with an iPhone app). The code runs fine on watch simulator and iPhone but on a real device there's a delay. Upon searching here and there, it seems that the bug would come and go every so often (introduced WatchOS 6 or before, fixed WatchOS 6.1), even the delay matches (10, 13, up to 15 seconds) "historical data".