Targeting Stone Titan's Heart with melee is not consistent with other mobs

[Go_Urself]

What went wrong?
Could not target Stone Titan’s Heart when behind. It’s also difficult to target in general with melee skills.
I use Shatter Strike for damage and ward generation, but when I used Mana Strike to generate more mana, there seems to be a targeting issue. It doesn’t occur anywhere else in the game that I’ve seen so far.

Here’s a video of one attempt where I went behind and could not use Mana Strike skill.

Last Epoch targeting issue with Stone Titan's Heart

Thanks!