What that tells you is... they don't understand those interests.

Or they don't understand that their proposed solutions don't sound likely to solve the interests.

Suppose class mobility is good for poorer people. Canada has more class mobility than the US. Canada also has universal healthcare (like most developed countries) which helps people take risks like starting their own business which helps with class mobility.

Why do poor republicans in the US keep voting against universal healthcare?

They actually don't. You get that result only if you exclude non white people.

Overall, if you define working class or poor economically, they are more likely to vote for those things.

They probably still hate those conservatives though. Even now you can see the hate being directed to Blacks or Hispanics that may have voted for Trump.

One must consider the major confounding factor here that Republicans have made a substantial effort to publicly taunt and insult liberals from positions of power[1], and that being the target of that kind of thing can easily override what would otherwise be more compassionate interaction.

[1]: One example: https://twitter.com/CawthornforNC/status/1323813315169165313

This is because the heuristics low-IQ conservatives use are actually better than the low-quality first-order reasoning midwit liberals use. Policies that immediately enrich poor people at the expense of economically productive people are bad for poor people in the medium-long run. Low-IQ conservatives do not explicitly understand this, but their inherited heuristics encode it.