#i'd say this isn't as likely tho since mike is the only one with lines

[you-were-alone-too]

the only way that flashbacks have been utilized in the show is when a character is processing their trauma/grief, and the flashbacks are used to give us more context into a character and their relationships (like in season 1 with hopper and sarah, in season 3 with joyce and bob, in season 4 with el and the hawkins lab massacre, and most of the vecna visions)

also, according to the casting call, mike is the only character with scripted lines, meaning we're most likely getting insight into mike's internal thoughts/feelings OR vecna is using the memory to taunt mike