Row Height issues

On the Ribbon's Home tab, select Format. Look for the item Autofit Row Height. This setting seems to be causing your trouble: it automatically sets the row height to the size of the cell's contents.

I don't like this setting for two reasons. One is that it doesn't work properly. It will add height to a row to accommodate one extra line of text if the text in it is nearly - but not quite - the cell's width. It also has great difficulty - or perhaps I have that difficulty - working in merged cells and accommodating indents, not to mention both of these at the same instance.

The other is that it's hard to know where the format is applied. For example, if I apply it to one cell in a row, the height of the entire row will change, of course. How do I find the cell so formatted? There isn't an indicator anywhere. But after setting the auto-height I might set a format that contradicts it, such as setting a fixed height, in another cell. In such cases my autoformat is removed without notice and I have no way of checking the status.

And neither do you. Try selecting the entire sheet and apply a fixed row height. If you have macros in your project check whether they apply autoheight.