That sourness is because the milk has gone bad. If you leave the snow lotus milk in the air overnight, it will turn sour.

The following is from Baidu:

Snow lotus is a traditional Chinese medicine. It is different from food. It has its own indications, dosage and dosage restrictions, and is not suitable for long-term use. Milk left at room temperature for a long time may ferment and go rancid and be contaminated by bacteria. Drinking such milk is not good for your health. Secondly, this method of taking has not been recorded in the literature and does not conform to the conventional usage of snow lotus. The traditional way of taking snow lotus is to soak it in wine or decoct it in water. Third, the effects of snow lotus soaked in milk on the gastrointestinal tract are widely circulated, which is inconsistent with the main functions of snow lotus in dispelling wind and cold, relieving spasms and relieving pain. In addition, snowdrops generally cannot grow in low-altitude areas, which means that except for the suitable areas on the western plateau in my country, it is difficult for snowdrops to survive in other regions.