@Igor Zysko Welcome to Microsoft Q&A, Thank you for posting your query!
It could be a portal issue, can you logout and try again(try in different browser or incognito mode) and let me know the status. I was able to repro the scenario in my lab and able to add metadata, even I was able add the metadata in Storage Explorer tool.
Hope this helps!
Kindly let us know if the above helps or you need further assistance on this issue.
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