Linking shared library with libm

I have a program which is linked (dynamically) with libm. There are also several plugins for this program. Plugins are loaded explicitely with dlopen(). Some of these plugins use round() from libm.

On one system (Linux Mint 19.1 gcc 7.5.0) the program does not work because of unresolved round.

Here is simple example:

1. Library (lib.c)

    #include <stdio.h>
    #include <math.h>
   
    void func(double a, double b)
    {
        double c;
        c = round(a + b);
        printf("c = %lf\n", c);
    }
   

2. Main program (main.c)

    #include <stdio.h>
    #include <dlfcn.h>
   
    void *dll;
    void (*f)(double, double);
    double a = 1.234, b = 4.321;
   
    int main(void)
    {
        dll = dlopen("./lib.so", RTLD_LAZY);
        f = dlsym(dll, "func");
        f(a,b);
        return 0;
    }
   

3. Building (Makefile)

    all:
            gcc -Wall -Os -shared -fPIC lib.c -o lib.so
            gcc -Wall -Os -rdynamic -fPIC main.c -o main -ldl -lm
   

4. Run on Debian 8, gcc 4.9.2

     ./main 
    c = 6.000000
   

5. Run on Linux Mint 19.1, gcc 7.5.0

     ./main 
    ./main: symbol lookup error: ./lib.so: undefined symbol: round
   

Now, add -lm for dll compilation

    gcc -Wall -Os -shared -fPIC lib.c -o lib.so -lm

     ./main 
    c = 6.000000

So, the question is - why on this particular system one must use -lm not only for main program but for plugin also?