Hello @ Mushroom , this thing is happening to me, @ Blaja and many other players too. I had like 89/100 supplies and 50/100 stock before I went afk for the night, just came back in game and found out I've 9/100 supply but stock are only 59/100. It happened to me before too and many times.
https://imgur.com/QFDqJVB - I've noticed that this siren is on and glowing when this thing happens.
https://imgur.com/lI4QPuY - this is me in the morning, took this SS in the middle of resupply, I did 67/100 and had 59 stock already made, didn't sold any of it yet.
https://imgur.com/4fLlReD - this is me after 5 hours, 54 of my supply got used and it gave me only 18 stock (instead of 27), I'll try to get another SS with accurate supply next time.