Hi Simon,
Please excuse us for the inconvenience this regression has caused!
It appeared that one of the improvements in the latest version does not work well with multiple editors! We fixed the issue and it will not happen in the next R3 2021 release.
Since the release is due in September and we have three months till then, you can temporarily fix the issue by disabling the StripDomEventAttributes filter or downgrading the Telerik.Web.UI.dll assembly to R1 2021.
Best Regards,
Rumen
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