So i bought a new back unit today and started upgrading it as it wasn't upgraded at all when i bought it and i noticed as soon as i started upgrading it it changed to TEC Defense instead of the Striking defense i bought it with. Is this a regular thing? a known bug? is there a way to change it back to striking defense? I have not delved too far into upgrading/augmenting etc. as not super invested in game yet.
It did not change defense, as (almost) all Units have MEL-, RNG-, and TEC-Def; what is shown as the "primary stat" is just whichever one has the highest value. You can see the total of each of these on the second tab of the Unit's description box.
In your case, it probably had an Augment that boosted MEL-Def to make it the highest stat, but by enhancing it, that minor amount of extra MEL-Def was overshadowed by the natural amount of TEC-Def the Unit gained, thus causing the main displayed stat to be TEC-Def.