I have a block with a select field with two options ('default' & 'mega'). It contains a text field, which is set visible when the 'default' option is selected ('visible' => ['block_title_style', '=', 'default']). When a second instance of the block is added to a page, and the first instance is in 'edit' mode (i.e. not in 'preview' mode), the second instance is inheriting the conditional logic of the first instance.
To put it simply, when both instances are in 'edit' mode, and I toggle the select field in the first block instance, the text field is visibly showing/hiding in both blocks on the page.
Thankfully, this isn't an issue when the first instance is in 'preview' mode, otherwise it would create a big problem as you would not be able to configure the second block instance differently to the first.
The extension MB Conditional Logic uses jQuery to check the field ID and value to match the condition. So if there are two or more IDs that are the same on a page, the condition will apply to all blocks or not work properly. You can move the edit mode to the sidebar to avoid this issue.