I've made a new version found at the link below where I've reduced the number of exploding fuel tanks in the bomb to about 1/3 of the original since the new game version apparently can't handle that number of explosives. I hope this one works better for you and everyone else.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3013513559

For some reason the nuke exploding (which worked fine in 1.2-1.4) does not approve of the new way explosions are handled in 1.5 and I get massive and I mean massive FPS drops that last for upwards to 30 seconds before it subsides. The Bockscar nuke works just fine, but this one has a few more parts and just grinds the game to a halt now, and in some cases the lag seems to cause the plane to explode. I'll try to reduce the number of explosive fuel tanks in this one sometime soon. Great way to update the game to once again utterly break stuff that worked before... 🙄

It shouldn't and it doesn't for me. However I get some extreme lag when this bomb explodes but not with the Bockscar bomb for somr reason (I'm so sick of all the random ass bullshit that happens in this game for no apparent reason) so maybe the lag spike somehow affects the plane for you. Sorry but I have no idea why it behaves like this now, it has never been a problem before so I'm guessing it has something to do witht he new way the game handles explosions. Clearly it's handling this bomb's explosion like absolute shite... Sorry I can't be of more help.

why does this explode when bomb explodes

I'll slowly but surely tweak all of my planes to work better in 1.3 but there are quite a few so it may take a couple of weeks to get them all done.