I had this exact issue with my new (to me) 2018 ATAS after only a couple of (wet) days of getting it - it started but stopped when I let go of the switch unless I increased the revs for a minute or two. First it was once in a few times but seemed to get worse until it was every time.
I bought the bike from a dealer some distance away and was not keen on going straight to the local dealer as I've learnt in the past that they use the "let's keep changing things until we strike lucky" method of diagnostics.
After reading lots of threads on here and other forums I stripped the switch housing apart, cleaned the contacts with contact cleaner, tweaked them slightly with a jewellers screwdriver to ensure they made good contact and gave them a light coating of ACF50 to try and prevent the issue returning.
Started first time afterwards and has done so every time since - probably over 50 times so far.

There is an official recall for the start switch button issue. Just apply it.
Sometime opening the switch and spraying it with contact cleaner can help, but it's not the solution.

I had a similar issue on my 2017 DCT, I initially sorted it be rocking the starter switch through all 3 selections half a dozen times with the ignition off.
I think it's dirty / corroded contacts in the switch gear. Works for a while then you need to repeat.

There is dirt in the switch cleaning it fixes the issue.
You can pull apart the right switch cluster (two screws) you will have to undo the front brake lever mounting bolts (two bolts) and move that out of the way first. with the switch cluster in two parts remove the small return spring now before it flys around the room. Then undo the start switch screws (two) that keep it in the housing, turn it over and give it a really good spray with brake clean while working the switch. Let it dry then give it a spray with CRC 226 (dielectric spray) or the like your problems will be solved. This is far better than a new switch from Honda and takes much less of your time.