OSX 10.14.6, late 2013 13" macbook pro.
I keep a lot of tabs open on my laptop, and lately I've started noticing that I can't find tabs that I thought were open. Turns out that when I close the macbook's lid, it will sometimes close whatever tab or window is active at the time (equivalent to cmd+w, I think). If, for example, it's an unsaved Word document, when I open the laptop again I'll see the "do you want to save your work" dialogue; if I had Chrome or Safari open in the foreground, it'll just close whatever tab was open.
This doesn't seem to always happen, making it frustratingly difficult to debug.
Any idea what could cause this behavior?