When using the entity autocomplete field the default value is not set on the block configuration form.
Steps to reproduce:
Create an entity with a Taxonomy reference field (multiple values)
Create a views block and add the field to the exposed filters
Enable the filter in block
Add the block and set some values for the filter field
Now edit the block again and the field value is now "Array"
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Apply views_block_placement_exposed_form_defaults-3158789-3.patch, if $exposed_filter_values[$exposed_info['value']] is empty, it will throw 'Notice: Undefined index' error in dblog. The new patch can fix this error.