I mostly agree with the rest you typed but this excerpt makes no sense at all.
Your personal feelings about this arc aside, even with you calling out that these are "deliberately (established) tension and frustration in the characters" (it clearly is deliberate if one reads these chapters, though I did not see a comment saying it was by accident..?) it boils down to the quality of what is delivered (which is what most peeps here criticize). And it is mid af to say the least.
To directly work into that statement, how is this all "established" (as you typed)? We went from the cosplay finishing, barely any interactions between the two, into her cosplaying into her knowingly neglecting her partner and their beforehand established plans of meeting friends.
Then her noticing her partner being clearly in distress (this was all before any calls or the possibility of her going pro/viral) but deciding not to communicate with him at all the whole day for no reason shown to us readers.
Timeskip to them leaving, the whole day being her actively avoiding him, keeping that up on the train home, keeping that up afterwards and then acting like she did the two most recent chapters. Completely out of character. With no setup at all.
When was the establishing? Do you mean us and to a certain extent Marin getting to know about his complexes like 50-60 chapters ago? Their dreams 90 chapters ago? Their feelings and characters grew, developed, changed, all that stuff.
Taking random facts that were thrown at us and some other characters to get worldbuilding and characterization going is not establishing whatever happened these past ~10 chapters.
Also, your last sentence is just pretentious rambling. These are NOT complex challenging feelings at all. They became those to a certain extent AFTERWARDS, when her going viral happend, when she met Non and Gojou after being a stuck up B. When she had time to think it through.
She went from "I am shooo in wuv with my hub" to "I won't even try talking to the guy I love being at his worst because of some unexplained reason". She knows SOMETHING is wrong. Her character AT THAT TIME would've made him her first prio. Again, she did NOT know about the pro etc. shit. She also still doesn't know it is about his complexes.
This it not me commenting on your personal feelings about this arc, btw, not at all. These are factual things happening in the manga, whole characterization being thrown out in 2 random chapters, drama that has no place due to how characters actually are but is enabled due to how out of character they behave currently.
Also not doing this for fun but I just can't enjoy someone belittling, insulting, flaming, bashing etc. other commenters (like you did here) for more than valid opinions. You did bring up some good points, why accompany them with actual toxicity?