I've played around a bit more with that and did some more more tests.
Your reply regarding DataFrame explains what Maple does with Excel files during conversion. But that just applies to the data table itself, not the cells within.
I've tried to apply text functions to an imported numeric datacell of the Excel file, and it clearly shows that it is not a text, but a number. The "SearchText" function wasn't working, while a simple multiplication did.
When using this number however as a table index, something is different, as the number of decimals has influence on the content.
Just did a quick test with a matrix as well. In that case Maple doesn't allow any decimals at all, even if they are 0.
So apparently it comes down that that problem again.
I need to convert the input from Excel - which is a number with decimals (all .0) - to a whole number. Those can then be to index the table.