Now you know why we have a MBS Xojo SQL Plugin 
Anyway, I could offer to try/debug this a bit. e.g. make a sample project and we may check if there is somewhere an implicit commit.
The problem is solved using MBS SQL Plugin!!
There are 4 databases my project uses (two for environment 1 and two for environment 2: three of them works fine when I set AutoCommitOff, while one doesn’t work.
I deleted it, and recreate, and now all works fine!!!
MBS SQL plugin is good, so today I will bought it (and I want to study it quite well).