I'd still call it a GMS problem, personally. If it can be corrected with extra CSS, gms should be doing it on its own.
I meant that it's not a problem specific to GMS and would also happen with other non-GMS games.
It happens because when playing browser games on a mobile device that's the way the OS/browser handles the "long press" touch input.
Fair point that they could add the CSS to the exported project, but based on that post I linked it only works for a couple of browsers and there might also be people who want the functionality for whatever reason, so either way someone gets upset.
Easier to let any end user who wants it disabled to spend a minute adding in the CSS code to fix it for the couple of browsers it works for, but if there was a catch-all 100% working solution for every browser then I'd agree with you that they should add it - and they likely would.
Maybe adding the CSS (or a YYG approved solution) to the docs would be a good idea to save having to scour the internet for the information.