Fix (more testing required)
HI all - joined this forum as I have exactly the same issue, in my case on a new Canyon Endurace Ultegra 12-speed.
On my bike the issue appears to be not with the 21t cog itself, but with the 19t cog below it. When in the 21t cog, some of the teeth on the 19t are very close to the outer chainplates and this causes the chain the hang briefly when engaging with the 21t. There's two sets of teeth on the 19t that cause this issue, hence the twice-per-revolution ticking noise when riding.
I fixed this by filing a bit off the back of the offending teeth on the 19t cog using a needle file with a smooth back (so as not to damage the 21t cog). For now this appears to have worked, but I'll need to test some more yet.
I have photos etc. but am new to the forum, so can't post photos or URLs. Message me I guess (?) and I'll send you a link to the piccies.