This fault occurs in the Documentation Page for Thunder Packs Mega Modal Stack.
It occurs when I used my iPad Pro (12.9inch), but not in my MacBook Pro.
I opened the two example modals, first left and then right example. When done in that order, you can no longer reopen the left example, only the right. This error seems to only occur on the iPad. Refreshing the page restores everything, until you again select the left and then the right modal, after which the left example refuses to open.
Following your steps I can’t seem to replicate this on my iPad, whether in portrait or landscape mode. I tried not only you steps but also different orders of tapping the buttons and more. 
Indeed, odd. I just tried it on my daughters iPad Air 2. No issues to be found. It only happens on my iPad Pro 2 (12.9 inch, model MPA62X/A). I don’t have a second pro to test on, so I don’t know if it’s the form factor or just my iPad glitching out. But it is repeatable.
Not sure as I don’t own an iPad Pro here. Only thing I can suggest is perhaps, and I know it sounds odd, but you might try clearing the cache and then restarting the device.
Maybe if someone else out there has an iPad Pro and can test it that would be helpful to see if it is a device anomaly or something else. That said, that will be a hard corner case for testing here since I obviously don’t have that hardware. 
Does this happen if you build a page yourself using the stack as well?
I’ll check back on this thread a bit later, or to be honest probably in the morning since it is well after the end of the work day here.
Also, let me know what version of iOS both of your devices are running.
Restarted, Cleared and also upgraded to iOS 12.1.4. The problem was originally found on iOS 12.1.3
The problem still exists. I haven’t purchased the Thunder Pack yet as I was just exploring to see if I might find it useful.
Hmmm… not sure on this one. Without a way to test for this very specific device, and without knowing anyone who owns one myself, I may not be of much help. Maybe someone else on the forum with the same device will be able to give it a test and we can narrow down whether it is the device as a whole or your specific device. That said, the hardware is running the same software as other tests that seem to work fine, so it is a bit strange.
And this happens in both portrait and landscape mode?
Yes, both portrait and landscape. As you say, we’d really need another user that has the same iPad test this. Otherwise, I’ll treat this as a specific problem and have the iPad checked out with Apple. Cheers.
I’ll continue to look around for someone who can offer up a test, but until then we’re stuck I’m afraid.
OK. So some of my awesome beta tester friends have now tried this out… we’ve test on a regular iPad (mine as well as @bdekok’s) , an iPad Mini (by @Tangerine62) and now also an iPad Pro 12" (by @Steve_J) and all have returned the same results – we could not recreate this bug. That isn’t to say that it isn’t a thing that you can recreate, but so far we’ve not been able to duplicate it here, and I so far cannot think of what, code-wise, could be a cause that would standout on a particular device.