Should be fixedSome unintended behavior I found with Tera + Adaptability.
Since Adaptability and Terastal both increase STAB to 2x, GameFreak made it so that these two have a significantly diminished effect when combined, instead increasing power to 2.25x, which only raises the STAB modifier by 1.125x versus a second 1.33x. This aspect is reflected accurately on the calculator.
However, if your Tera Type does not match one (or both) of your original Types, you are supposed to lose the Adaptability boost entirely (per Bulbapedia, Serebii said nothing but I assume it's just not been addressed yet). This is implemented properly on Showdown after some quick testing, but the calculator does something else.
(1)Adaptability, No Tera
252+ SpA Choice Specs Adaptability Dragalge Draco Meteor vs. 0 HP / 0 SpD Mew: 382-450 (112 - 131.9%) -- guaranteed OHKO
(2)No Adaptability or Tera
252+ SpA Choice Specs Dragalge Draco Meteor vs. 0 HP / 0 SpD Mew: 286-337 (83.8 - 98.8%) -- 75% chance to OHKO after Stealth Rock
(3)Adaptability with related Tera Type
252+ SpA Choice Specs Adaptability Tera Dragon Dragalge Draco Meteor vs. 0 HP / 0 SpD Mew: 430-506 (126 - 148.3%) -- guaranteed OHKO
(4)Adaptability with unrelated Tera Type
252+ SpA Choice Specs Adaptability Tera Poison Dragalge Draco Meteor vs. 0 HP / 0 SpD Mew: 334-394 (97.9 - 115.5%) -- guaranteed OHKO after Stealth Rock
(5)Adaptability with Dragon Tera Type on a non-Dragon Dragalge
252+ SpA Choice Specs Adaptability Tera Dragon Dragalge Draco Meteor vs. 0 HP / 0 SpD Mew: 334-394 (97.9 - 115.5%) -- guaranteed OHKO after Stealth Rock
Some quick math with the damage rolls of 2 and 4 reveals that the 1.125x modifier to bring 2x to 2.25x is being incorrectly applied to unrelated Tera Types. I have almost no programming experience, but it appears the issue might have something to do with Adaptability's 1.33x Damage modifier being set to 1.125x for both original Types, instead of just being applied to the Tera one while being negated for the other. The situation with 5 seems to be related but I am struggling to wrap my head around the exact issue. Again, did testing and Showdown works fine.