^Exactly. The warp-speed saucer separation in "Farpoint" was an emergency measure to get the saucer away quickly so the stardrive could do a 180 and face down Q. The fact that they repeated the maneuver in "Arsenal of Freedom" was because it was cheaper to reuse the footage from the pilot; La Forge had taken the Enterprise
safely away from Minos, so there was no real reason for him to replicate a dangerous maneuver before going back to fight. And the saucer didn't need to escape in BOBW because that was part of Riker's plan. So really, we've never seen a saucer separation as intended "by the book".
As to the Odyssey
not separating, that's due to the real-world fact that at the time, the six-foot Enterprise-D
model was likely being refurbished by ILM for its appearance in Generations