Background: I have a Circle of Spores Druid with a Moon Sickle +2. Base spell attack modifier is +8 then the Moon Sickle adds +2 to druid spells for +10 total.

Issue: Chill Touch only shows +8 to hit where other Cantrips have +10. I believe the issues is that Chill Touch is not being treated as a Druid spell (since it normally is not) but Circle Spells are supposed to be considered Druid spells for that character.

Relevant sources:

Moon Sickle: "...+2 bonus to spell attack rolls and the saving throw DCs of your druid and ranger spells." Tasha's Cauldron of Everything pg.133

Circle Spells: "If you gain access to a spell that doesn’t appear on the druid spell list, the spell is nonetheless a druid spell for you." Guildmasters' Guide to Ravnica

It's a limitation of the system. 

You can customise the spell to add the bonus to hit / dc.