[With the caveat in Imma's comment]
I hold the following combination of beliefs, which are individually relatively common but rarely held together:
As a corollary: I have heard many people (the example that springs to mind is Peter Singer in his interview for the 80k podcast, but there have been others) say something along the lines of "I would support open borders, but high immigration leads to the election of right-wing populist nativists who will enact harmful policies, therefore I reluctantly support immigration restrictions." I contend that it is not the mere presence of people of different ethnic groups that causes right wing populism, but rather the aforementioned problems (e.g. cultural clash, crime, stress on government services, collision with government-imposed inflexibility in the housing and labour markets etc.) combined with left-wing denials that the problems exist (and insistence that anyone complaining about them is a deplorable racist) that drive voters towards the only politicians offering to solve the problems. Therefore, interventions to engage with and reduce the problems caused by migration ("keyhole solutions" in Open Borders lingo) could enable more open borders without a political backlash.
Related: In October 2017, "What important truth do very few effective altruists agree with you on?" was asked in the main Effective Altruism Facebook group and got 389 comments. (This is Peter Thiel's contrarian question applied to EAs.)
Calling them "truths" goes too far for me.
I have beliefs that very few people agree with me on, but I would not call them "truths". Some of them are not beliefs about objective facts, and even those that are about facts, I am less than 99.9 percent sure that they are true.