I have the same problem using Excel 2007. I don't believe this is related to
compatibility mode as I routinely have this problem when using all native
..xlsx worksheets.
I have 20 worksheets that all use the same template - each has 12 or more
chart tabs, all custom named. About 50% (or more) of the time, these
spreadsheets open with all the chart tabs renamed to "chart1" etc. Strangely,
it seems to be consistent in that some of my spreadsheets always open up with
their custom names, and some will consisently lose their names. I have not
documented any cases where the default names come up and then sometime later
the custom names magically reappear.
Since my spreadsheets all use the same template, I have taken some of the
"good" ones (where the custom tab names remain) and saved copies of them,
updated with appropriate data, and then seen them lose their custom tab names
on a future open. To be clear: I have one spreadsheet that opens with "good"
tab names, I save two copies of it with different data tables in one tab (but
nothing else changed, all charts are dynamically derived). One copy will keep
opening with custom tab names, and one will not.
This is a very annoying bug that seems specific to 2007.