I think the problem you have there is that "99900000000" is not a valid answer as the right hand 9 is not permitted in that column (for multiples of 8!). It's restricted to be at most 8 otherwise (as you have here) you get 9 * 8! which is of course 9! which then should be added to the value in the column to the left of it instead (that then gives you 10 * 9! or an extra 10! which finally gives you just 10 * 10! which is then the answer "A0000000000").

The testcases actually contain such example: 36288000 can be represented both as "99900000000" and "A0000000000". Only one of the solutions is considered correct though...

Can you provide an example of such a number?

I had a similar first impression, but thinking it through I changed my mind. However, I have not yet proved it to myself.

There's more than one way to encode a decimal this way. This kata should make it clear which one should be used.