Welcome to the forum. The freeform chat format was removed as an option when AI Studio was updated around the time of Google IO. Other people have complained about some of these changes, for example here Your opinion on recent AI Studio changes
At the time, it was announced that a backend script would update the no longer supported freeform saved chat files (at a later time, which Google did not specify).
Hope some of this is helpful. I am conjecturing that the upgrade script failed while processing your freeform prompt and truncated it. A Google engineer might be able to restore the previous version from a backup, if you manage to contact them.