I think it's bugged.  It's always the same tiles, so it's probably a bug in the map.  Like, they forgot to set that tile as traversable. It is pretty annoying.  You think you've got your strategy worked out, but no, that flat, open tile is inaccessible for some unknown reason.

Some of the long narrow tactical maps have tiles you can move to... you just need to zoom in a certain amount to "look past" the surrounding trees, hillside or buildings...

Definitely needs a tune-up.  The squares to the black widow's right could not be moved to until I zoomed in the map.

Another example, the tile right under my selected hero is  inaccessible

This is for .8, but the square in the upper right from the selected unit (between it and the enemy unit - the latter just above the ophidian) was not passable. I could not target the special spear attack through it or move into it, and there is no visible reason why this square should be impassable.