I realized that ever since the minecart changes from 24w33a, minecarts can't push other minecarts. The issue is now furnace minecarts don't push anything when the gamerule "minecartMaxSpeed is too high. Doesn't work all the time.
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Thank you for your report!
We're tracking this issue in MC-275291, so this ticket is being resolved and linked as a duplicate.
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Relates to MC-275347