I have mixed feelings about this. On the one hand, the incident is weird, whichever way you consider it. On the other hand, I could find no concrete evidence that foul play was a possibility. And add to this the well-known fact that next of kin are usually in a state of shock when a loved one passes and doubting the official version is a normal reaction, especially when the victim supposedly died of suicide.
I know very little about Taiji's life during or post-X, but from what I could gather, he was a rowdy free spirit and his life kind of went downhill after he left X, despite the great opportunity we was handed when they asked him to join Loudness. Things can easily go downhill very fast when you're young, rose to fame fast, made bad decisions with your money, etc. According to Yoshiki, Taiji's hit a rough patch shirtly after he left X. In his book, he mentions him having lost all his teeth and his unrecognisable looks, compared to his glam stage image when he was part of X. I have no evidence to back this one up, but I reckon that he might have been on some heavy ass drugs. If this portrait is accurate, a mental breakdown while under pressure is definitely not off the table. Fun fact: there are SO MANY suicides in detention cells they won't even let you keep your shoe laces. Being locked up in a cell with no support can do that to your brain.
TL;DR: I lean towards the suicide thesis, but maybe the authorities fucked up the investigation or maybe they handled his arrest the wrong way from the start (i.e. He hung himself with a bedsheet. They're not supposed to put fabric bedsheets in a detention cell. A blanket or paper sheets but not bedsheets that can be used as a slipknot). His fiancée and next of kin are devastated and questioning the official version is their way of coping with it.