Daily used spreadsheet loses 3 months of data

I use a spreadsheet to track all my health and exercise information, accessing it daily, each year starting a new one. I completed 2024, saved it, moved it to a folder like I always have done. My 2017 iMac saves everything to my iCloud drive. Yesterday I opened this spreadsheet to view some information and all data that I inputted back to October 1st was gone. I searched my local drive, my iCloud drive and only the one version which the data disappeared comes up. I was on the phone for 45 minutes with Apple support, they could not find the current file or any copies of the saved file. It was not on their server.???? The revert to previous spreadsheet version was greyed out, not accessible.

This spreadsheet was saved, closed and opened (successfully) literally 100's of times. I use many financial and other spreadsheets and have never had this happen in 40 years. I also back up this computer with Apple's Time Machine. It turns out that it is only backing up system data, none of my icloud data, because that is "safe" in the cloud.

If it wasn't saving the file, why would it open up successfully everytime with the new data entered? And if it opened up on December 28th, all intact with all data successfully, where is that file? Isn't it backed up on the cloud? Are there any temporary files I can access.? The apple support person was useless, after 45 minutes just said that it happens sometimes. Maybe I should access the web based icloud and check everytime I save a document.