it has been my common sense that FMA should have to change to VNAV ALT if there is any difference MCP target alt with FMC VNAV crz alt.(in VNAV mode)
If that were the case then any change in the MCP altitude would cause an FMA change to VNAV ALT.
The MCP has very little to do with "smart" concepts its an ENTER/DELETE button effectivly, VNAV ALT results when the MCP prevents the FMC/VNAV from doing what it wants, seeing you stepped down to FL349 the MCP didnt inhibit the change so it stayed in PTH, try stepping to FL351 and it'll go VNAV ALT as the "dumb" MCP is preventing it from going there.