This behavior is probably known as it is the same as release version.

But posting is just in case.

If a unit has 4 movement and wants to join an ongoing combat 5 tiles away, the UI will incorrectly show that the unit could join.

In reality the unit will exhaust it's movement points right next to the combat area and be unable to join.

The UI calculation needs to be improved to take into account 1 more movement, not just adjacency to combat area.

This is rarely an issue, because once the combat starts, the combat area will unpredictably expand and envelop the units anyway.

Still, the units will have lost 1 turn of combat (the first strike), and in tight situations this might lead to losing the fight.

In the picture: archers cannot join the combat (0 movement), even though the UI arrow is white instead of gray.

G14 - Mongols Turn 185 - Join Combat Bug.ctr