Perhaps a browser extension, or AV setting, or proxy server or something like that.
I tested in different machines. Some instances of Firefox, of course, have extensions. But none of the Edge instances.
These are the browsers on a "virgin" machine with a Windows 10 preview with nothing else. But this machine is a bit older, so I still found the original Edge.
This one works okay:
* Edge 44.18.362.449.0 (EdgeHTML 18.18363)
I was able to delete the "result"-field a dozend times, either with DEL or BackSpace, each time the "Try-It"-button filled it correct as desired again and again.
..... Here comes the "no luck" section: ..........
Installed FF on this preview box, it behaves as I reported. Once "result" got deleted, it never got filled again, except after refresh / F5:
* Firefox 85.0.2 (no extension)
My developing machine with a recent Chromium-Edge and also no extension:
* Edge Chrome Version 88.0.705.63 (Offizielles Build) (64-Bit)
Chromium on a Raspberry, also no extension installed by me. However, "uBlock Origin" was installed by default:
* Version 72.0.3626.121 (Offizieller Build) Built on Raspbian , running on Raspbian 9.13 (32-Bit)
* Internet Explorer 11 - TryIt doesnt do anything. No wonder why. I tried just for completeness sake.