I am continually frustrated too by this issue. I use the Windows client as well as browser-based (Firefox) version on both Ubuntu Linux and other Windows machines and have the following issues:
often losing indentations after using browser version then later using Windows-client version
sometimes Tab will indent in browser version and sometimes not (so have to use buttons)
sometimes indentations in browser version will be preceded by spaces and sometimes not
As a premium member I am also a bit disconcerted that when I checked the forum here and noticed others with the same issue there was no official responses or acknowledgments. This is not an obscure issue but a core part of the functionality!