Bug present on 6.46beta59, might be present on stable, can't test right now, but maybe someone else can.
Later edit: bug was introduced in 6.46beta28, 6.46beta16 is fine, stable is fine.
There's some error with the default value for Signal Strength Range, and the validation for the values entered is wrong too.
Using WinBox, creating a new ACL defaults Signal Strength to "-1..120" which isn't quite usable, and it doesn't accept anything outside that range that can be usable.
Using WebFig, creating a new ACL defaults Signal Strength to "9223372032559809000..120" which again isn't usable, and the only value it accepts as input is either "-" or "-..-" which again, isn't usable in any way.
From Terminal you can set whatever value you want though, in the -120..120 limits.