Split Sheet and Freeze Split are two separate operations. Unfreeze (untick Freeze menu option) should only remove frozen nature of the split, not unsplit. This is encountered if you wish to adjust the frozen split. LO forces you to reset both splits then refreeze them. This interrupts workflow and wastes time. I recall MS Oriffice behaving as I expect. Seems like a easy improvement to implement (he said ignorantly!!!) Confirmed in current STILL version, 6.4.7 and I remember it at least as far back as 6.0.7
Split and freeze are basically a three-way toggle. If you are on Freeze, then why don't you just select Split Sheet instead? I think that does what you want to do. You will notice that when you select split, then freeze is unselected.
Closing as RESOLVED NOTABUG based on comment 1