Apparently avoiding the ambush is never guaranteed. I used to believe that it was 0% chance of ambush if you stealthed day 1, but yesterday proved that assumption wrong.
As someone who enjoys stealth, I've probably played Framing Frame more than any other mission in the game as it's a fun mission and decent xp & cash. I usually solo it until I've got a lock on day 3, as playing with randoms online almost always results in nearly immediate failure of stealth. Last night, an assault happened to me two times in a row after stealthing day 1. Both were pro jobs with a 3 skull "Overkill" difficulty. First time it happened I was solo, then after failing that on day 2 when a cloaker kicked me in the face then downed the AI teammates who tried to come help (dun dun dun), I joined a group of 3 others doing FF. In both instances, I sprinted out of the doors of the train car as soon as they opened trying to get to the construction workers & get them down but an assault popped as soon as I made it through the doors of the large building you have to run through. I also planted a jammer in both instances (an old habit from one playthrough where one of the construction workers had run down to the end of the street and I didn't find him to tie him up before he had called the police), but I've done that many times before and never had it result in an assault.
Long story short, it is possible to get an assault on day 2 after stealthing day 1. Seems a very small chance, but it absolutely can happen. I'd advise bringing at least one of your "loud" guns just in case even if you stealthed 1. Just don't forget to switch back for day 3!