If you have done a lot of work and choose "save" (command-s) and if you are too quick to then choose "save as" (command-shift-s) and save a pdf, the original save seems to be cancelled, but the file has no indication that it is not saved.
This means that when you close the file, you get no warning that the file needs saving, and it just closes. When you then open the file, it is the old version of the file that opens.
This problem has been recreated on two different machines, so it is definitely a bug.
Mac OS Big Sur 11.6, intel based machine
Illustrator 25.4.1
If you have done a lot of work and choose "save" (command-s) and if you are too quick to then choose "save as" (command-shift-s) and save a pdf, the original save seems to be cancelled, but the file has no indication that it is not saved.
This means that when you close the file, you get no warning that the file needs saving, and it just closes. When you then open the file, it is the old version of the file that opens.
This problem has been recreated on two different machines, so it is definitely a bug.
Mac OS Big Sur 11.6, intel based machine
Illustrator 25.4.1