The issue isn't the game being pushed back, it's the complete lack of communication. If someone has given you money for a product you have promised them, then the appropriate thing to do when faced with a setback is to let your customer know of said setback.
It doesn't have to be an extensive, detailed thing. Even just a "Hey guys, due to COVID we are facing medical/personal/unforeseen issues, so the project is on hiatus until further notice." That's it, just a sentence. Maybe a follow-up when you have a more concrete idea of your timeline.
Just basic, simple communication. It is perfectly reasonable for someone who has paid money to be annoyed or upset when they get NO communication about the thing they paid for.
I understand that they've been having difficulties, and I would by no means expect them to prioritize a game of all things, but if you're going to take on the responsibility of accepting peoples' money, you need to also take the responsibility of communicating with them.