Check the chain tension with it on the largest rear cog.
It could be that your derailleur can't take up enough slack if the chain length was right for a 42T (or whatever) outer ring at the front.
It may just need to lose a few links.
I run one of my bikes as a 1x8 and don't have any problems.
Sounds like your chain is too long. Break the chain and run it around the granny gear in the back and the front chain ring, without going through the derailer. You should have enough links to complete the chain, +1 more. Any more is too many.
Many people also use a jump stop with their 1x8 or 9 systems to help keep the chain on.
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