I have reported this and discoved the reason too.

My steps are as follows:

BIAB

1. Generate song in BIAB (110 BPM)
2. At bar 130 slow tempo -15
3. Generate and save song

RB

1. Open song in RB
2. The realdrums will not be generated correctly

One the following happens
2a. Song's realdrum track is not generated fully
2b. When drummer closes the point that tempo is slowed the drumer gets totally crazy playing not with same tempo as other band does. The drummer tries to slowdown the tempo before that bar and this especially sounds crazy.

You can test this one very easily. Do not forget to slow tempo and make other tempo changes in song.

Thank You For Your Time
B