Not a bug
BYOND Version:512.1461
Operating System:Windows 7 Pro 64-bit
Web Browser:Chrome 71.0.3578.98
Applies to:Dream Maker
Status: Not a bug

This is not a bug. It may be an incorrect use of syntax or a limitation in the software. For further discussion on the matter, please consult the BYOND forums.
When you increase the number of frames, the new frame appear with a copy of the last frame you had, instead of a clean slate. Gif:
Lummox JR resolved issue (Not a bug)