External SSD windows boot drive no longer booting

Hi! I installed Windows 10 on an external SSD and have been running it on my 2020 MacBook Pro. I recently upgraded to OS 11.4 and suddenly I could no longer boot windows from that drive. I restart, hold 'option' and choose the external. It appears and is recognized as a boot camp Windows disc but when I select it to boot, it shows an apple logo, then a pop up says the version of Mac OS needs to be reinstalled on that drive and naturally doesn't boot up in windows as it used to.

Anyone else have this problem, or have any ideas what might have changed or if there are settings to adjust? I have the security settings to medium, but otherwise don't have many ideas and have done a fair amount of reading to discover the problem but so far to no avail. It was a complicated process getting windows working on the SSD in the first place, and I dread the thought of starting over. haha! Any help is appreciated. Thanks!