This wasn't a problem unique to your tablet, it was a problem with how Edge was interpreting some JavaScript in the forum software.
Can't say I'm too surprised.
In general trying to do a WYSIWYG edit box from inside a web page is ... problematic. I've never seen a system that handled it well across ALL browsers; with the ALLEGED break from legacy IE that is Edge it just introduces a new pain in the backside to an already painful situation. Much like every other time a new flavor of the month browser engine comes along, its sending everyone who's JavaScript is filled with cross-browser fixes scrambling to make sure that the existing code even works, much less isn't silently throwing hordes of errors.
One of the reasons I chose SMF back in the day when I was the one making those decisions was it DIDN'T use the type of code that would break cross browser moving forward... but that was on version 1.x. Version 2 didn't deliver on nearly any of the promises, introduced code I knew was fragile and unreliable -- and now with the 2.1.x beta they're pissing their own bed by adding jQuery? DERP.
As it is, I'm on the hunt for a new forum software for my own projects BECAUSE of issues like this, and I'm quickly finding that most of the off the shelf answers -- PHPBB, myBB, vBulletin, Xenforo, etc, etc -- are either rotting on the vine from neglect, making things less reliable and usable by blindly mixing technologies, or simply screwing things up with mind-numbingly ignorant frameworks' like bootcrap or jquery. Kind of makes me glad I'm NOT stuck dealing with the mess that is version 2.0.x of SMF. PARTICULARLY since the latest upgrades makes all legacy skins incompatible -- while not TELLING you that. That was starting to be an issue when I left back six years ago... I can't imagine it's gotten a whole lot better. There were a LOT of problems I lacked the time or resources to fix, and wouldn't be surprised if they just sat there unaddressed this whole time.
The web is in a period of transition, it's a hard time for every site owner. I really think it's going to come down to a full "dotcom burst" style blowout to get things back on track. It's always been a house of cards... but lately it feels more like a game of Jenga that's gone on for WAY too long.