I have issues with some sites not displaying account login options and the only way forward is to manually disable pi-hole.
An example is www.boots.com. I have tried white listing www.boots.com and boots.com, both as plain domains or with wildcards but each time one attempts to login, the screen refreshes but the login boxes are not displayed. Disabling pihole overcomes this and all is fine.
I only have a very plain 'out of the box' installation with default settings - no groups or other grown-up settings.
When this happens, go into your Query Log and click the dropdown to show All. Look for any seemingly unrelated blocked domains. Chances are that one of them is related to the missing part of the site. A common prefix used for these is "cdn" which stands for Content Delivery Network.
With your example boots.com, I also was missing the login box. In my Query Log everything was green except for an entry for cdns.gigya.com which was blocked. Whitelisting that and refreshing the boots page showed the login box. If you wanted you could then add a note to the whitelist entry to mention it's for Boots.
Looking at the page source, gigya is indeed referenced in the login box code, so that checks out.
A related effect I've seen is when a site pops up a box asking you to accept cookies and blocks access to the page until you've interacted with the box, but Pi-hole and/or a browser adblocker plugin prevents the box from loading or appearing.
The result is a page that seems to load okay but is unresponsive, can't be scrolled or clicked. Because the script on the page is waiting for you to click things on a box that you can't see.
The fix for this is to reload the page without browser adblockers running and see if a box appears. and then also find and allow any specific entries in Pi-hole if also needed.
