Initial observation :
Tried to test the 1800kg bomb in custom online arena (default settings, 'protect objects' disabled). Found I could not kill a BH on Bowlma or CraterMa maps with it.
Tried same with a formation of B17s and noted it took me much more bombs than usual to kill said BH. Switched to offline (Ndisles terrain was up), and the 1800kg destroyed a BH without problems.
Systematic testing:
Weapon: M4/75 75MM gun with HE only
Target: BH on a medium field
Tried CraterMA and BowlMA terrains first online, then offline. Instead of taking an expected number of 16 rounds to kill the BH it took 47 shots to take it down on both terrains, in both modes.
Then loaded NDisles offline. Here it took 16 shots, just as expected. After that, I created another online arena, with Ndisles as well. Now I Suddenly had to hit the BH 47 times before it went down. Switching back to offline for a third test, this didn't change anymore - still 47 hits required instead of the 16 it took earlier.
Did the same test with Mindanao terrain, with exactly the same results: First offline test - 16 hits. Going online custom arena - 47 hits. Going back to offline arena - still 47 hits required.
Putting up a map in a custom online arena permanently increased the hits required by a factor of 3!
Each time when I loaded a terrain in either mode, I made sure the object settings were still at their default value. The BH value was at 2.7813 each and every time.