I am back with my stupid short testing sessions results.
As I've got Morgan this event I thought "why not check this triple attack buff" and so I did. All percentages are taken from JP wiki.
Ares with her assist's boost (30% DA, 15% TA), one 2% DA accessory, Awakened Tit buff (50% DA, 30% DA) and Morgan's guaranteed TA buff.
32 attacks done: all were TA.
Got some additional samples for Ares with her assist's boost (30% DA, 15% TA), one 2% DA accessory and Awakened Tit buff (50% DA, 30% DA).
125 attacks done: 78 DA, 47 TA.
Also checked how Morgan's guaranteed TA interacts with Cagliostro's guaranteed DA and... results are quite interesting.
Only 11 attacks done: whether DA or TA occurs depend on which buff was applied last, so if Cagliostro's buff was applied second it was always DA, if Morgan's buff was applied second it was always TA.
It may very well be possible that Morgan's and Cagliostro's buffs work differently than "guarantees combo attack" buffs (at least Daphne's buff's description is different than those two).