I think it's a bug, or more perhaps more worded more precisely: it's a result of inadequate user interface portrayal of the symbol properties and their effects. My deduction is that the same dialog box for point symbol properties is re-used, but not all the controls there are applicable for Simple Line Symbols.
It would help to also add an additional graphic showing what the desired final complete line will look like, even a pencil sketch. For example, are you really after a marker line, where every x distance has a symbol, or is it just arrows at the middle and/or ends?
For a true symbol marker line, you need to use Cartographic Representation, which requires Desktop Advanced license. If you're fortunate enough to have that, see how to get started with fix symbol rotation to page.
Rich man's (CartoRep) example: equally spaced fixed rotation marker symbol.
Poor man's repeated marker line symbol: it works, angle is fixed at 45deg to page according to symbol properties, but the distance between each point varies.
To round out the examples, for arrows at ends like you'd use to depict stream flow, see Line Properties >> Line Decorations and rotate to follow line. Here we've created a complex symbol, two arrow types on one line, by stacking lines (see "Layers" panel).