Body Summary field showing literal ampersand character code in view
This is to confirm and clarify the issue according to my findings.
1. I was able to reproduce your findings by using a fresh Drupa 8.7.3 install and after importing your view in which you used the {{ body__summary }} rewrite.

2. I was able to reproduce the error by enabling Display any HTML as plain text option in Basic HTML text format.
3. I was also able to reproduce the error by rewriting the view field output and choosing {{ body__value }} which basically prints the raw value of the body field.  

My opinion is that 2 &amp; 3 are normal behavior and it should be expected that in these particular cases the raw HTML is displayed. However, in 1, it seems a bug because the summary shouldn't be displayed as raw HTML; it should follows the same rules as the selected format.
I suggest to open an issue in Drupal issue tracker telling them about 1; maybe there is a reason behind this behavior, or if somebody already knows the answer, please elaborate.  

Regards