ISNUMBER() - Shows for cell formatted as number FALSE

Issue:

• Cell C1 fetches a number from a web page via IMPORTXML.
• Cell B1 refers to it (=C1).
• B1 and C1 are formatted as number and also display numbers.

However, ISNUMBER for both cells is FALSE and thee shown decimals in B1 and C1 cannot be changed. Even if I copy the format of a cell D1, where ISNUMBER returns TRUE, to B1 and C1, it remains FALSE there.

What could be the reason if a cell is formatted as a number, a number is visible there (via formula) and yet ISNUMBER returns FALSE?

Thanks in advance for your help!