Just a quick FYI: Ran into the same boot troubles with 20190824, but simply reverted to my last working version, which was 20190815 in my case. Simply did it with TWRP / Install, navigate do [root]/data/lineageos_updates and choose the latest working version.
No problems down the line, but I do not use gapps
I just did the update (via updater) for 20190825, same result: system is patched, tries to boot into system but end up starting twrp after a couple of minutes.
I’ll gladly test any new packages, if this is of any help…