Startup issue while booting Windows from external drive

I have a 2017 Macbook Air, and I have installed Windows on my external hard drive so that I can boot my Mac into Windows. All that works fine, but the issue (which arose a few weeks after I first dual booted) is that when I try to boot back into MacOS Catalina, I get a black screen with a circle and slash.

I am, however, able to boot into MacOS after I run Recovery Mode and choose my internal hard drive as the startup disk. The irritating part is that I have to do this every single time I run Windows and want to switch back to MacOS. Is there any way I can fix this permanently? Also, is this potentially harmful in the long run?

PS: I have selected my internal hard drive as the Startup Disk within the Disk Utility app on MacOS as well, but that does not fix the issue.