Is wearing a flag patch on a backpack against regulations?

Yes

I would say yes but I honestly don’t know the answer.

It is technically, but I’ve never had a problem with it.

Yes

Yes

Apd.army.mil

...

Flag is going into combat

In all honesty I don't know , while I was on active duty btwn 1980 and 1984 we didn't wear flag patches on our BDUs.

You should look up the US flag code. I bet you had the same flag as worn on the right sleeve. Positioned so that blue field with the stars was to the viewers right? If so, that is against the flag code because basically unless it's on the right side of a vehicle or uniform the blue is to it's own right which is the viewers left. So many people are wearing the reversed flag patch (the name is a clue!) on hats, chests, backpacks, etc incorrectly.