iMac keeps shutting down instead of sleeping

There is no macOS 18.6. Also knowing the exact model of the Mac is greatly beneficial as well, plus knowing what version of macOS you had installed prior to the change in OS.

Run the third party app EtreCheck and post the complete report here so we can get the correct system details and also examine the report for clues.

How to use the Add Text Feature When Posting an EtreCheck Report - Apple Community

You may also want to review the following article as well:

Writing an effective Apple Support Communities Question - Apple Community

There has been a lot of users who've had problems with the sleep function and recent Macs and systems.

They've done what many of us have:

1 - turn off sleeping the mac:

2 - set screensaver to kick on at a specified time and turn off the monitor as a pre-determined time.

3 - shut down the Mac when leaving for any length of time and overnight.