Issue logging hex - preceding the colon with '0' and logging two uint64_t values prints the first value twice.

[gimrWay]

Issue logging values in hex where preceding the colon with '0' and logging two uint64_t values prints the first value twice.
Problematic code:

SPDLOG_INFO("Hello world: 0x{0:X} 0x{0:X}", val_a, val_b);

I preceded the colon with '0' by mistake, and I quickly realised that it works as expected if you format it without:

SPDLOG_INFO("Hello world: 0x{:X} 0x{:X}", val_a, val_b);

But it seems odd to me that preceding the colon with '0' does not throw an error and yet produces this result. Other values are rejected. Is this expected behaviour? What does preceding the colon do?

[tt4g]

The {fmt} library is used for the log message format.
{0:X} means that the value of argument ID 0 is output in hex format.
See also: https://fmt.dev/11.1/syntax/