HI,
I have just run into this problem. It appears that because wordpress theme updater is very simplistic that if another theme has the same name and/or folder and a more up to date version it will try to update.

Steps to replicate issue:
1. Download purchased theme
2. Install Genesis theme
3. Install Oasis
4. Turn on Oasis theme
5. Refresh backend Apearance
6. An update to Oasis appears

To stop this happening I just updated the theme’s folder with a different name which fixed the issue however if there is ever an update to this child theme it wont show up.

See this stack exchange about theme update simplistic. https://wordpress.stackexchange.com/questions/344504/if-i-build-a-custom-theme-will-it-update

Other issue is that the onboarding for this theme no longer works like it used to