I thought there was an issue in TLS handling but it doesn't reproduce with this code...

main.c

#include <dlfcn.h>
#include <stdio.h>
#include <stdlib.h>

typedef int (*get_foo_t)();

int main() {
    void* lib = dlopen("./libtls.so", RTLD_LAZY);
    if (!lib) {
        printf("%s", dlerror());
        exit(1);
    }
    get_foo_t get_foo = dlsym(lib, "get_foo");
    if (!get_foo) {
        printf("%s", dlerror());
        exit(1);
    }

    printf("foo = %d\n", get_foo());
    return 0;
}

Makefile

.PHONY: run
run: dlopen-tls-test libtls.so
	./dlopen-tls-test

.PHONY: clean
clean:
	rm -f dlopen-tls-test *.so

dlopen-tls-test: main.c
	cc -o $@ main.c

libtls.so: tls.c
	cc -shared -fPIC -o $@ tls.c

result

% make
cc -o dlopen-tls-test main.c
cc -shared -fPIC -o libtls.so tls.c
./dlopen-tls-test
foo = -1

tls.c

static __thread int foo = -1;

int get_foo() {
    return foo;
}