Sorry, but either you weren't aware of the syncing issues, then, here you go, this is the explanation: Stop running the app on two devices in parallel. If you want to switch, sync, close one instance, and then open the other. Or you were aware of the syncing issues, then, you shouldn't have played on two devices in the first place and the point wasn't that this "issue" gets fixed but just to rant about this. (Besides, you experienced several times that you had issues and continued with this gamble?? I don't get it!)
Either way, can you play the same WZC match on two devices in parallel?? And I don't mean just have it open in two tabs, I mean deploy armies in one tab and then order movement in another without reloading and syncing? I believe not. And even if, WZC by design is turn based, so everything you do will always be synced at least once per turn, and if you lose progress you will at most lose one (uncommited) turn worth of actions. Hence, the technological basis wasn't designed with parallel play in mind. WZI doesn't have "turns". Therefore, "fixing" the sync issues "on the go" won't be that easy (or possible at all, for a one-man project). You're asking for a complete game rebuild at that point.
Sure, Fizzer should be more up-front that multi-devicing will eventually cause problems and how to sync (especially in browsers) if you still want to multi-device this game. But idle clearly - for some time at least - was a side project and was built on an existing technological stack of (hardware and) software. And even with idle out of beta, this is still just one mode of playing WZ.
You know the warnings on microwaves that you can't dry your pet in them? What you're asking here is similar to someone asking a microwave-vendor to enable pet-drying in microwaves. There is a warning on it that it can't, so it won't work. Hence, in WZ you can ask for a more visible warning, not that Fizzer over night completely changes everything he made in the last years.
PS: I know that not everyone understands coding, so, I get that for some portion of users it will be plain mind-boggling that WZ can't do "basic syncing" when every other game they play can. So, yes, I was a bit aggressive here, perhaps wrongly, as I don't know your knowledge basis. But, not every game has thousands of developers, not every game company is as large as Blizzard, Microsoft, Epic, you name it.
Why even have it available on both if it won't work correctly when using both?
Because some users will play only on one platform, but not necessary on the same. There are only-Android players, only-iOS players, only-browser players. You can't find a common basis for them all without excluding some players. Besides, can you write the same mail on several devices at the same time? So, why are there mail apps for desktop-PCs, laptops, and phones?