I asked a question to the Home Office regarding family members who applied from abroad, their response was as follows:

Originally posted by EEA Applications said

All EEA travellers can still enter the UK via the production of their valid passport or national identity card. EEA travellers are able to use eGates in order to gain entry into the UK if they are travelling on their passports as they were previously.

Those EEA citizens and their family members who have made an “in time” application to the European Union Settlement Scheme (EUSS) which remains pending after the end of the grace period will continue to be protected by the Citizens’ Rights (Temporary Protection and Application Deadline) (EU Exit) Regulations 2020. This means that they will continue to have a right of admission until their application is finally determined, and will not require a grant of leave at the border nor will they need to await the issue of a COA.

EUSS family permit holders may re-enter the UK as many times as they wish during the validity of their permit. Provided that they have applied to the EUSS before the date that their permit expires, they will continue to have their leave extended by virtue of section 3C of the Immigration Act 1971 until such point that their EUSS application is finally determined. In all cases, Border Force officers will be able to verify the existence of a pending application on Home Office systems, should they need to do so.