I’ve had this happen before. When two trains are waiting for the same block to clear, as soon as the train leaving the block exits they both get a green light and enter the block at the same time, resulting in a collision. Sometimes at a low speed so they don’t derail and you don’t get an alert.
Replace the block signals going into the many-to-one block with path signals. This will ensure only one train is allowed to enter the block at a time.