Hi Martin,
In the case of the 'Vintage Amp' voice on a T4, this is due to the Super Articulation aspect of the voice. Other voices also suffer from this issue.
If you load the voice then press the 'INFO' button (upper 8 ) on the selection screen, you will see the Information about the voice which includes the following:
Legato notes within an interval of a 4th sound as a hammer on, pull off, or slideIn fact on my T4, this only works if you play notes down to a 4th below the first note, but Yamaha don't mention that.
I personally think that many of these effects are overdone and become intrusive at times, since as you have found out, they usually involve a reduction in volume since the new note is not being retriggered from the start of the sample. There is a setting in the CONFIG2 Utility menu which allows you to turn off the SA2 Auto Articulation effects, but this makes no difference to this S.Art. voice.
Since there seems to be no way to turn off these effects on S.Art. voices (which would be the best way!), all that I can suggest is that you try not to play legato anytime you don't want this effect to be triggered, but how many of us can play a fast riff totally cleanly?
Certainly not me!
Regards
Ian