Edit 2018/12/31: BluSpark has released updated TWRP which fixes this issue. Whilst official TWRP has not fixed this issue, please use the custom version from BluSpark instead. See https://forum.xda-developers.com/oneplus-5t/how-to/guide-how-to-dirty-flash-beta-t3882416 for more info. The BluSpark TWRP build offered is compatible with both OP5 & OP5T.
Original story: It seems that the automatic update installations (feature for root users) are broken since Open Beta 22 (5) and Open Beta 20 (OnePlus 5T). You will likely see the start screen of TWRP when using this feature.
If you tap the bottom-right icon (logs), you'll see a red "Zip Treble compatibility error" message on your screen.
This error is caused by the TWRP recovery version for the OnePlus 5 / 5T being incompatible with Android P (Dec 19th, 2018). Please wait until an updated version of TWRP gets released.