Pokémon GO developer Niantic has confirmed that an issue affected the most recent EX Raid Battle invitations. In the below statement, the developer also apologizes to players affected by it:
Trainers, the most recent EX Raid Battle invitations were not as wide-reaching as we had hoped. We discovered an error that executed an older version of code, which did not include the fixes we’d previously introduced. We’re sorry for the confusion, and we will make sure that the next round of EX Raid Battles appropriately executes the improved code.
Source: Official Pokémon GO Facebook