Perhaps that allocator could print a warning message if you're not deleting the last element (when built in debug mode). That would make it a lot more clear how that kind of allocator should be used.
I use this pattern a lot and my allocators print a huge warning when they detect this kind of leak. +1 for this suggestion. It's a hard bug to track down in nontrivial code.
FixedBufferAllocator is meant to minimally viable like in settings when there's no shared concept of "printing" or an OS for that matter. Check out LoggingAllocator which can take/wrap the former.
Maybe put the word "Sequential" in the name (like FixedBufferSequentialAllocator) to really hammer it down that you can't randomly delete. Then also have a movable head pointer so you can still deallocate in either reverse or forward order, it will still successfully free everything.