Had the same thing happen on a CH20. I put the valve seat back in and it "seemed" like it fit tight. It ended falling back out on me next time I used it.
I ended up taking it to a engine building shop and had a oversize seat put in and a valve job done, head resurfaced, for something like $50 or $60.
I have never had a problem with it again.
I believe they will fall out because of the engine running hot because when I pulled mine apart it was past due for cleaning on the heads.
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