@Pathoschild reported an issue

SMAPI checks for mod updates by comparing the manifest.json version against the latest version on Nexus. Unfortunately 2018.05.22 isn't a valid semantic version, so SMAPI won't tell players when you release a new version. (More specifically, leading zeros aren't allowed in semantic versions.)

You can fix it by either removing the leading zero like 2018.5.22 (SMAPI will correctly show 2018.10.22 as newer than 2018.5.22), or using an incrementing version like 1.2.

Many thanks for pointing that out.
Dunno how I could miss that, will be fixed with next version :)

Fixed & closed.