This is because of the animdata cleanup in rBd2e55cb. This works ok in
general, but causes issues with the localized node trees used for compo/
shader/texture previews. These localized trees share the same default
action as their original trees, and then remove fcurves when removing
muted nodes (which should affect the localized tree only).
node_free_node_ex now has an argument for disabling animdata cleanup,
which is also not necessary when freeing the whole node tree (because
animdata is freed in advance anyway). In addition to that it also checks
the NTREE_IS_LOCALIZED flag to prevent freeing of fcurves in the action.
f70921d90c