Currently EE4 strips all tags from the description and there is no option to disable this, however the description field used in the tooltip is filtered so if you are comfortable with PHP you can use the FHEE__EE_Datetime_In_Calendar__to_array_for_json__description hook to change the current description for your own without stripping the tags.
Your filter is currently using strip_tags on a description that already has all of the tags stripped out, you need to replace the description and then set your allowed tags.
You can just use the same code the calendar uses to build the description, only without stripping tags, for example:
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