Cell data types revert to previous

I formatted a column to 'date & time' but found the sum formula resulted in an error. So I changed the column cells, all of them, to 'duration'. Then the fun began.

When I deselected and reselected a cell, it had reverted to 'date & time' and it did so repeatedly. I couldn't make it change to 'duration'. So I deleted the column, formatted a new column to 'duration' and re-entered the numbers. Momentarily, the sum calculation worked, but then as soon as I clicked on a cell it changed to 'date & time' and the sum cell produced an odd result. Then I looked again and the cell was changing my entry '0:40' to '0.01', with the sum cell changing to match. After many attempts I failed to make the cell retain my '0:40' entry.

This is quite bizarre!

I copied the cells containing narratives and deleted all the rows I'd been working on. Then I pasted the narratives into new lines, formatted the next column to 'duration' and manually typed in the entries. Magically, by the time I entered the sum formula at the bottom of the column, the cell types had changed themselves to 'date and time' and nothing I did would make them change back to 'duration'.

What's going on? This feels like Microsoft, not Apple!