"layout autosave on" works only for explicitly named layouts in .screenrc.
Namely, if you specified some layout as ABC, switched to it within Screen session and detached, this last layout will be restored after the session is resumed.
If you did not specify any layout in the configuration file, you have to run internal command "layout save your_layout_name" before detaching.
This is a .screenrc with predefined layouts to choose from:
0 — one (the only region)
1 — two-v (the screen is splitted vertically into two regions, default layout)
2 — two-h (the screen is splitted horizontally into two regions)
3 — three (three regions — two in the upper part of the screen and one underneath)
4 — four (four regions)
Once selected from within the Screen these layouts are preserved upon detachment until the next resuming of the session (due to "layout autosave on" command).
If you've built your own layout it must be saved with "layout dump filename" command. Then you can use a code from "filename" file to add your layout in #layouts section of this .screenrc to select from.
By default "two-v" layout is loaded with the only window with bash.