Unable to rename sheets in Numbers on Mac mini M4 and iPad Pro

I'm on a Mac mini M4 (But the same thing happens on my iPad Pro).

I have a Numbers spread sheet for monthly expenses.

I duplicate the sheet each month in the same document.

I try and rename the sheet but after pressing return it reverts to the previous name!

This was working fine all last year and Jan and Feb this year but now the sheets are named Feb-1 Feb-1-1.

Obviously I want them as March and April, but it won't let me!

I click on the name and use the drop down menu to 'Rename'. Doesn't work.

I type it in the right hand column 'Sheet Name'. Doesn't work either!

This has been brought up a few times in the past on this forum but there is no answer.

The Apple support document just tells me to do exactly what I am doing, so no help!

Does anyone have a solution please?

TIA Ian

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