The reason is most likely that you are using the same Grosh account on two devices with different operating system (iOS / Android) users. See Is the Grosh premium subscription tied to a Grosh account?
When you share the list with your spouse, so he or she gets an invite code that can be enter when creating a separate Grosh account in the app at startup (log in and select have an invitation).
If your spouse logs in with your Grosh account on his/her own phone with another Android or iOS account, the operating system will report back that there is no Premium subscription in the operating system (Android or iOS) and hence the Grosh account is downgraded to the regular free version as part of the login process.
