(this was supposed to have the tag Wikidata, but for some reason that no longer shows up as an option in the tags field)

Steps to reproduce:

• Click "add" to start adding a new statement
• Select a string property (e.g. Sandbox-String)
• Enter some text (e.g. 12345) as the value
• Click "add reference"
• Select a URL property (e.g. reference URL)
• Paste a URL
• Delete the URL (leaving the field blank for now)
• Change the value of the string (e.g. to 54321)
• Paste a URL in the reference field

Expected behaviour: All fields are correctly filled out, so the save button is enabled.
Actual behaviour: The save button is disabled and the statement can't be added. The only way I've found to re-enable it is to delete the reference and add it again.

There might be a way to reproduce it with fewer steps, but this is simply what I happened to do to trigger it in the first place. I could reproduce it multiple times in Vivaldi and an old version of Chromium on Wikidata itself and test.wikidata.org.