Got this working by using the same WKProcessPool for all the webviews.
First create a process pool once somewhere:
processPool = [[WKProcessPool alloc] init];
Then use it when creating WKWebviews. The pool must be set in the init method, not afterwards.
WKWebViewConfiguration *config = [[WKWebViewConfiguration alloc] init];
config.processPool = processPool;
webview = [[WKWebView alloc] initWithFrame:frame configuration:config];
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