Looking at the string value of the property and this document https://learn.microsoft.com/en-us/openspecs/exchange_server_protocols/ms-oxcdata/c33d5b9c-d044-4727-96e2-2051f8419ab1 I found that:

A contact entryID must start with flags 00.00.00.00 (4 bytes) followed by provider (16 bytes) FE.42.AA.0A.18.C7.1A.10.E8.85.0B.65.1C.24.00.00.followed by version (4 bytes) 03.00.00.00 followed by type (4 bytes)04.00.00.00 followed by type Index (4 bytes) 00.00.00.00 followed by EntryIdCount (4 bytes) 00.00.00.00 followed by EntryIdBytes (EntryIdCount bytes) the remaining 4 bytes seem not relevant.

Indeed the string value of the property includes these mandatory values for a contact entryID and the EntryID I was looking for sits where it is supposed to be. Just did not see it before.

Thanks for pointing it out for me.