Maybe focus on remaining electric baseboard. When one level was replaced, did it cause a complication? Is 'other level' remaining overly hot?
Shut down baseboard by breaker if possible and leave doors open so efficient furnace could temporarily heat whole house? Then check usage.
Could a neighbor be tapping in by extension cord?
Shut down baseboard by breaker if possible and leave doors open so efficient furnace could temporarily heat whole house? Then check usage.
Could a neighbor be tapping in by extension cord?