But your question was why was his account of the date ever in doubt.
The answer is he wrote about it fifty years later in his 70's.
Did he have the date tattooed on his leg a day later? Did he get the date from an old newspaper he had lying about the house?
While he may be certain about his recollection of the date there is reasonable uncertainty about the certainty of a 70 year quoting a date from 5 decades past.
The question of the date currently has something like a one month window plus/minus .. so there's general agreement that it happened in the later part of 79 BC.
> For him to get the date off by an entire season is quite a stretch.
In my own experience it's not at all uncommon in the septuagenarian, octogenarian, and nonagenarian crowd to be off by that much in their stories.