This is expected behaviour. This allows the user to not perform the action if they pressed it by mistake (for the mouse up not triggering).
This is a bit weird, and I guess it has something to do with how Ignition embeds images into buttons?
Before going any further with this, have a good read of this thread: Very Concerning Read/Write Issues. Important information for all users
Implementing momentary presses can be hazardous depending on how they're used.