hi Cress,
a workaround consists to add a double-length -1 image-layer (empty) at top of your timeline,
and set the AnimLayer post-behavior to "ping-pong",
on your exemple, anim-layer = 50i, empty image-layer = 99i
(drag the right-handle while reading the length up to 99 in the timeline top-bar),
so a preview play in loop mode would plays indifinitely your anim in ping-pong mode.