"Enter Once" has no trading logic. It is responsible to set a max count of trades per period (ex. bar, as in this case).
FSB can open at maximum two positions per bar - one long and one short. When you use Enter Once, the program opens only the first signal.
The problem with the backtest of the above strategy comes from the particular combination of Price Move entry, the SL, and the TP. This causes the "Pessimistic" algorithm to take wrong decision for the entry point and the protection. The algorithm announces an ambiguous bar in such case.
When the bar starts, the program sees that there are two entry points with corresponding SL and TP for each of them. These are 6 potential trading points. The FSB Pro algorithm tries to compose a "pessimistic" route between the points without looking in the future. This works in 99.9% of the cases, but some times it doesn't. Fortunately, FSB Pro comes with a Comparator and a Monte Carlo tools and reveals such problems easily.
If you think, such complex algorithms are drawback, imagine how MT tester works. It simply draws and follows a jagged zigzag line between the OHLC points and calls that "Tick mode". No ambiguous bars - no problems for the traders.