Ahh, indeed, @magpie I think you’re on point!
Just one note: you don’t have to manually navigate in Safari, there’s a little icon in Slack when a webpage is opened inside Slack’s webview to open the same page in Safari.
From a similar thread (iOS 11 unable to install IPA of public install page - #2 by tamaspapik):
Another alternative: when the page opens inside Slack click the “open in Safari” icon at the bottom right corner to reopen the page in Mobile Safari.