I don't know what's happening there i flashed this OS more than 10 times and switched to many ROMs for testing, still i myself using same kek-ROM,
Now read below instructions as i do:
1: Flashed stock ROM via msm tool 10.3.7
2: Unlocked bootloader
3: Boot to twrp Slot B Because msm tool flash ROM only in A Slot,
4: Copied, ROM.zip, Gapps, Kernel & Magisk in USB OTG.
5: I stayed in Slot B while TWRP and connected OTG and just flashed ROM. zip
6: Flashed Twrp installer.zip
7: Reboot twrp from Twrp. Now i am in Slot A
8: Flashed Gapps.zip
9: Press wipe and just swipe to wipe data, factory reset
10: Reboot to system Setup everything
11: Now reboot tp twrp
12: Flashed Kek-Kernel & Magisk
13: wipe Dalvick/cache
14: Reboot system ... All OK
I got as far as step 9 (leaving out 8), but then it drops back into TRWP. Also the earlier instructions had the same result, but dropping me in Lineage recovery.
I am using a OP 6T and lineage-kek-17.1-20210322-OFFICIAL-fajita.zip for this.
If someone encounters the same problem: the problem is the misleading description of Step 9:
This: 9: Press wipe and just swipe to wipe data, factory reset
Should be: 9: Press wipe and press Format data and enter `yes` to format data and factory reset
Then it in Step 10 display the Nethunter animation, but still, it does not really boot LineageOS. This process either drops into TRWP recovery or just loops the "Nethunter loadding" animation forever.
As I didn't know what else I can do about this, I continued with the step description.
This results in Step 14 in the "fiery Nethunter" animation during boot, but, alas, the animation loops for quite some time and then the device drops into TRWP recovery.