I just updated the core and all layer plugins with a new ‘Framing’ parameter, since it was a small change.
This now allows you to use Single-image PNGs as animation frames on spritesheet battlers (default for new and newly edited flipbooks). Make sure to load the flipbooks layers after anything else that animates battlers to give them priority!
You can also set it to Unchanged (it’s a drop-down) to keep framing effects visible for that flipbook. That includes the default boss collapse animation, for example.