There are 4 options I can see:
1) See whether there are any theming options (including night modes or similar as well as any actual theme options LG provide) in the device settings which have an affect in this. I don't have this device or any LG so don't know what options there might be, but I can imagine some device theme/display setting interacting badly with your message setting, in which case changing that (or even toggling it to something else and back again) might correct this.
2) See whether there is anything at all in the app's settings that changes this (yes, I know you've looked, just trying to cover all possibilities).
3) If the problem is due to an incompatibility between old (Android 8) setting and LG's "new" Android 9 system or apps then deleting those settings (i.e. resetting the app to defaults) might fix it. So you could start by force stopping the message app and clearing its cache, or clearing its cache and rebooting. More likely to work is clearing the message app's data. Unless LG are doing something very weird and non-standard that should not affect your messages: there's a system database that stores those independent of which SMS app you are using, so clearing data for the message app should only affect the message app's settings. If you are worried then back up your messages first (probably not a bad idea anyway), but I've cleared data for both system and third party message apps on other phones without losing messages (on my phone, running Android 10, it's the system app "Phone and Messaging Storage" which actually stores the messages. On other phones I've owned there was a separate "Message Storage" app. So don't clear data for any app that has "message" and "storage" in the name, because that will erase your messages).
4) I know you said you didn't want to, but the final option is to use a third party app. Personally I think there are several which are better than any manufacturer's own app that I've ever used, so I never use the built-in app, but it's your preference. But if nothing else works, this will.