First I must clarify that I know it's intentional game design for weakened unit try to run away to preserve themselves when facing overwhelming pressure.
Although I may not love it, I admit it's a reasonable game mechanic and can benefits players too.
Bbbbbbbbbbbbut- sometimes it really goes annoying.

I remember I have read somewhere, that unit speed and experience have effect in a unit's chance of retreat. So a unit have higher speed and experience have better chance to retreat instead of being destroyed.
Not sure where I get this information from, forum post or manual? I forgot.
I still notice so many cases that my 5-star medium tank fail to chase down an 2 strength infantry unit.
I don't know why... can I just pray for a good RNG or is there any factor I don't know that can improve the chance of destroying enemies' unit?
May be terrain play a part too? It feels reasonable if my tank can't chase enemy through forest or marsh...but than I find they can't chase down enemies on open ground anyway, enemies still run in all direction like you just smoked a rabbit hide...