My mom has a dining table and a coffee table, both purchased new in the 90's, from different manufacturers, that started to get light watermarks a few years ago. The marks actually form very quickly, such as condensation from the bottom of an empty ice cream bowl - moved 15 minutes later. This did not happen when the tables were new. I knew polyurethane wouldn't do this, so I did some research.
I had noticed many furniture flippers on YouTube using a spray on laquer instead of polyurethane because it dries so much faster and does not require as much time to cure. At home polyurethane needs two weeks to cure, in a factory, it goes into a low humidity/higher temperature drying booth to cure for several hours. Lacquer does not require so much additional time. So most manufacturers switched to lacquer.
But eventually, the lacquer crazes into thousands of tiny microscopic cracks which allows the moisture to wick up underneath it- causing the white/light marks. (On older furniture, white marks are typically the result of moisture absorption into/under wax or polish buildup.
I have successfully touched up the areas with markers. Were it my furniture, I would give it a light sanding and wipe on 2-3 coats of polyurethane. Some Youtubers claim this doesn't work, and the lacquer will still pop off and continue to crack. But my parents are in their 80's, this only needs to last a few years. I would just like to prevent something from getting left long enough to blacken the actual wood. However as my mother isn't physically able to do things herself, she becomes infuriated when I offer to do anything aesthetic. If I can do it behind her back, she is grateful afterwards. (I just can't figure out a way to hide those tables for two weeks! :thinking:)