Nope. Still won't leave Vnav Alt in that situation.
The only thing i can think of is G/S capture. Was this an ILS?
GS will always do it once you capture regardless of MCP alt selection - I refer to it as a "hard" mode.
However - if you are inside of the capture region of the
VNAV approach logic and in VNAV ALT with a lower selection in the MCP - the modes will transition to SPD/lateral mode/VNAV PTH and fly away from the previous VNAV altitude once it captures the descent profile. Things such as VNAV speed intervene whilst remaining in VNAV PTH mode and the ability to set a higher MCP Altitude and remain in VNAV PTH mode are now enabled.
Of course the usual caveats apply such as using an approach from the FMC database that is compatible with the VNAV approach logic.
Works pretty well on GNSS approaches
