With the availability of 19.07.0 RC images, please help the development team by using this thread to report images that don't save their settings.
If you can, please include the output of
df cat /proc/mtd
using the preformatted-text button
These are "safe" commands to show what file systems are mounted and their sizes. See https://openwrt.org/docs/guide-quick-start/sshadministration for instructions on how to use SSH if you are not familiar with it.
Please also check