I have been running 11 speed Sram eTap with 53/39 chainrings and 11/25 cassette and 53 link chain length. I just built a new bike with Sram eTap utilizing 1X setup with 52 chainring and 11/28 cassette. I cut the chain for the 1X bike to 53 links, the same number of links on my 2X bike. On the 1X bike the derailleur will not shift to the 25 and 28 cogs with 53 link chain. It requires 55 link chain to shift upwards to the 25 and 28 cogs. . I am puzzled why my 1X bike requires 55 links when the gearing is similar to my 2X bike.
I assume the derailleur is adjusted properly because it shifts to the 25 and 28 cogs without the chain attached.