My LG 55" G3 is 3 months old and the questions apply rarely; but, will occur. In every instance I've been able to get everything sorted to my liking. I wouldn't call the occurrence especially dark; but, it can be dimmer than usual. Only suggestion at this distance, is to remember to check appropriate box to "use settings for all attached devices". Seems to assure more lifespan to the adjustment. Though I find the LG will require re-fiddling once in a while.
BTW, I start out with Brightness at 90 on a scale rising to 100. Match contrast, clarity, etc. to that as standard. Always leaves me enough fiddle room.
And, I admit...I'm an inveterate fiddler with tiny settlings. But, I've been so happy with my 55" G3, I'm planning on sticking with LG from here on out - after years of being devoted to Samsung and Sony. Especially with LG's leadership in OLED tech.