I am. I have been able to reproduce it but for me it is any boss fight. I took screenshots this last time to convince myself it was real. I had 692T after logging in and collecting offline ( I forgot to notice how much was collected offline), 697T after the boss fight (successful one hit), after changing floors it was 508T, which is approximately how much I had before logging out last time.
I've also seen it drop by upgrading items. That's what made me notice it and start paying attention. During this last boss hunt, I would log in and start to upgrade and suddenly I had used up 1M gold with only a few item levels (around 3K per upgrade). I actually caught it with the screenshots this afternoon.
Actually, While typing this, I went in to get version number, It happened again, dropped 2T when I changed floors, but this was not a boss fight. It changed floors right after I logged in and lost the gold.
iPhone 7 plus, iOS 10.2