What's strange about that?
Seriously? What have Republicans done for the working man in the last 30 years to earn their vote that hasn't enriched their overlords?
Taking away water breaks. Taking away mandatory hest breaks. Taking away tax credit for tools.
I suspect (although I haven't seen anything concrete) is that the perceptions were driven, not so much on "what republicans did or didn't do for the working man", but rather more on what democrats did support that indirectly cuts against some truckers' interests.
One can certainly surmise that the covid policies that democrats pushed for certainly didn't make the lives of Truck Drivers any easier.
A percentage of Teamsters driver tankers that deliver gasoline. Democrats pushing for the elimination of gasoline could certainly be concerning for people who haul gasoline. (not saying that's a good reason to not push to move away from fossil fuels...but it's a factor none the less. I'm sure members of the Asbestos workers union weren't happy when their industry got phased out either)
Teamsters represent more than just truck drivers, in some cities, they represent a large number of retail and hospitality workers.
Obviously the Teamsters aren't endorsing the republicans either...but it's not terribly shocking that they're declining to endorse Democrats this time around.
There are other countervailing interests at play besides just money/breaks/OT.
And that doesn't even delve into how some of the social policies can play a role. While some rural and southern Teamsters likely agree with the policies of Democrats on the economic front with regards to workers' rights. That doesn't mean they're going to agree with them on other things.
For instance, pretend you were a unionized widget maker in a widget factory, and I as a candidate am/was very "Pro-widget and pro-widget worker" on economic policy. That's all well and good...but if that's literally only one of the few points of agreement we have, there's going to be a certain threshold of how many other policies I can disagree with you on before you eventually decline to endorse me.
The promise of wages and working conditions support is only guy buy a finite amount of "look the other way on the other stuff" credit.
To use an extremely exaggerated example. If you were unionized...and of the two candidates, Alex Jones was the pro-union candidate fighting to get you a 5/hour raise, and extra PTO and breaks, and the other was a stereotypical corporate politician... would you endorse Alex Jones? Or would you simply decline to make an endorsement that election cycle?