Hero’s regain full health for no reason

Bringing video evidence would make things far clearer. In HvV the only way to regain health is by being selected as target, and in HS there's no way to regain health.

That is, if we ignore Passive Regen, Healing Cards and Bossk.

However, no character has 50% or more of his total HP as Passive Regen (the most regen is 46%, so a character that has less then 50% of his health won't come back to full health just by waiting for his passive regen to kick in, again with the exception of Bossk).

Healing Cards OTOH have 2 distinct groups (soon to be 3 probably): Heal on Kill and Heal on Hit. We also have Bossk's Healing Card, which is also an exception

Heal on Kill as its name suggests only heals on a kill, so in Hero Showdown this means a player can at most heal once, because if he heals twice, match is over as he killed both opponents, but in HvV there's no restriction and it's fairly common for heroes to come back to full health from almost dying. The most a HoK card gives is 100 HP, which is at most 15% of a Hero's HP (I'm only talking about heroes who have a HoK card here). So a crippled hero has to kill more then 3 enemy heroes and allow his passive regen to restore him if he wants to come back to full health from almost dying. The heroes who can do this are: Leia, Luke, Rey, and Vader

Heal on Hit is for now restricted to Palpatine and Iden. Their cards allow them to heal for each target hit by their left abiltities (Chain Lightning and Stun Droid). In the case of Palpatine he can heal at most 100 Health by using his ability on HvV, but 75 is more realistic as he doesn't need the star card which increases the number of targets he can hit with his Chain Lightning ability, and in HS he can heal at most 50 HP. Iden OTOH can heal 280 in HvV easily by using her ability, or 140 in HS. But since these are HoH cards, these characters can heal how many times they like in either HvV or HS.

Han, Chewie, Lando, Finn, Yoda, Maul, Kylo, Phasma, and Boba don't have any healing Star Card (soon to change with the October update).

So if any of these heroes, which don't have a healing card, heals more then 33% of their health (max regen value of this group), then something isn't right (except in HvV if they become Targets). But Phasma can increase her life with Temporary Hitpoints from an ability, so this can trick someone into believeing she regained more health then normal. Also Finn and Yoda have this ability as well, but theirs is actually applied to them and also to any ally near them, and they stack on each other, so all the heroes on the lightside can have temporary hitpoints through either finn, or yoda, or both.

Bossk is an exception to all of this as his Passive Regen completely regenerates his health even if he was left with 1 HP remaining. If that wasn't enough, he also has a card which allows him to heal 30 HP per second (instantaneous increments of 30 as each second passes) while he is within his Dioxis Grenade Area of Effect.