Thank you for sending the sample file.
Unfortunately, I was unable to replicate the problem you reported. I tried applying all sorts of styles (text color, fill color, font size, alignment) in different cells and ranges of cells. I saved changes to box, and to the Documents folder on the device, I tried SAVE and SAVE AS, and each time I reopened a file, my style changes had been retained. For this testing I used a Galaxy Tab3, and the SwiftKey keyboard.
I will mention that each time I reformatted a file and wanted to test the style retention, I did a FILE-SAVE or FILE-SAVE-AS to preserve the changes.
Perhaps I'm misunderstanding the problem you are experiencing. The only thing I can recommend is that you send me a step-by-step recipe of how you are working when the style edits are lost.
I'm sorry I can't give you a better explanation at this time.