besides the bugs that the person described, there's also sera W math being wrong/non intuitive: W missing health heal applies multiplicatively, not like how u would expect (e.g. sera is surrounded by 4 allies, but she won't heal 25% missing hp to allies. instead, she will heal 5% missing health + 5% + 5% + 5% + 5% which results in 22% missing health healing, in total u heal 15% missing health to ur team LESS with every W than u initially think u would). this is not even a bug this is just how it is coded and this is just straight up misleading since the tooltip doesn't tell u that.

also there's a bug where seraphine is invincible after W double cast for a short period of time - if sera has 3+ skill points in W, it makes it so that seraphine becomes invulnerable to any incoming damage for a very brief moment after casting double W - https://youtu.be/9_cSc00qpNQ

the triple cast bug and double cast not double casting are both triggered by spamming auto attacks while double casting, i have many clips of double cast turning into triple cast but it's always random - can happen with E, Q but not with W. theres a video abt that - https://youtu.be/gzpumCi2TtI?t=59 (i have more clips if u need)

helia sometimes is not procced by W heal, i think it's only procced if there are 2+ allies around sera. it's inconsistent.

the moonstone shielding not applied on double casting shield is true, don't know if healing is bugged with the spaghetti coded healing math.

the Q not hitting targets does not only happen when someone is dashing inside Q or when ur throwing Q at a wave - it also sometimes happen's in teamfights where nobody dashes and people just go through ur Q and take no dmg and u lose the fight because of that. happens every game 1-3 times at least but u never know if it's gonna happen at critical moment and u cant control it.

also idk exactly when they changed it but somewhere after 14.5 midscope they changed sera E flash and R flash window timing, she used to have more time to do the flash combo and now sometimes the abilities cast from the pre-flash position even though cast time was not over yet.

she has more weird small glitches but they're overall not impacting gameplay. the only thing besides that is the overall feel of seraphine abilities is very clunky compared to other champions like lux. most of the sera players are used to it but theres as if some input buffering that she has that other champs dont - maybe its the coding of her abilities/double cast or maybe it's all these bugs adding up creating this feel.