Has anyone noticed that the in app spreadsheet cannot open xlsm files, only xlsx files.

yes because Spreadsheet doesn't have macros
here's a full list of limitations from Ref Guide:

The OneStream Windows App Spreadsheet feature can perform most of the tasks accomplished by the OneStream Excel Add-in, with some limitations. These limitations include, but are not limited to:
• Macros
• Solver
• Document properties
• No option to insert copied/cut cells in spreadsheet
• Preview format does not update for format types Effects (Superscript, Subscript, Strikethrough)
• Ability to manage conditional formatting – color scales are reversed
• Shift+End
• Spreadsheet border button does not store last selection
• Text Direction option from Alignment formatting
• Underline formatting does not appear same as in Excel
• Not all Chart options are available in Spreadsheet
• Ability to change chart colors in spreadsheet • References to other sheets in Spreadsheet is a different process
• Ctrl+PageUp / Ctrl+PageDn correctly changes the tab but does not reset tab focus in spreadsheet
• CTRL+N creates a new spreadsheet, however if an existing spreadsheet was opened with unsaved changes, it closes the existing spreadsheet without saving edits made since last save

If I'm using XFGetCell and one of my references is pointing to a cell that is also a dynamic retrieve (i.e., XFGetMemberProperty), the worksheet doesn't seem to be refreshing properly.

I am finding some very odd things happening in Spreadsheet. In a CV in a Spreadsheet as a Form, when copying values from one cell to another, sometimes it does it correctly, sometimes it scales the number, sometimes it divides the number by the number of cells you have selected. No real rhyme or reason.

Another issue i saw with spreadsheets as from was copying null values.

Sometimes when you blank a cell and submit, it reverts back to the number which you wanted to clear.
was able to test this in 5.0 and the copying and pasting, as well was the clearing cells is fixed.
Has anyone noticed that these settings do not apply when embedding a CV in Spreadsheet?

Also, if I double click the column to fit to width it does not go to the end of the text. In the example below there is text to the right of ""Default"". The row header is spaced by 2 tabs so it aligns.