Loop Back Test

If they are out of phase they could sound more hollow or wider…but when summed to mono they will sound a lot weaker in this case.
See if flipping polarity makes them sound more natural/less wide.
But anyhow, it seems there is nothing unusual about the recording timing so it’s either just your perception of the delay that isn’t there or something else is causing it (accidentally set a track delay?..dodgy plugin?)

You then plug in the Tascam to another USB port on your computer and plug your headphones or monitors into it. In other words, the Tascam is only connected to the computer. You don’t connect it to the keyboard at all. The midi-to-USB cable is a separate thing connected to a separate USB port on the computer. All it does is send the midi data in.

The latency of the sound card shouldn’t even matter in this case because the line and mic are plugged in to the same interface. As others have stated, it’s just the difference of the mic distance.