you for reaching out on the community and telling us about your issue. I do apologize for the issue you're having. That is quite odd that they continually disappear even after remaking them. I think in this lets try to see if we're able to find anything with a super validate. To perform a super validate we'll go across the top to file > file operations > (HOLD CTRL + SHIFT) validate and repair > Super validate file > OK.
Once you've given this a try see if you're able to get anything in the log that might relate to the calendar. Once you get a chance let us know if you're able to find anything out.
a picture is worth a thousand words …
Can you please capture one or more images of the parts of your Quicken window
showing the issue, sensitive information blacked out as necessary to protect
your privacy but annotated to describe the situation, and attach the image(s)
save images to files of file type PNG,
JPG, or GIF only. They're easier to work with than PDF files.