No problem with 15.1 on an iPad Pro ... After right click Share > Air Drop on Mac and selecting my iPad - this pops up on my iPad and selecting BIMx opens the model as it should.
Does Air Drop of other files work normally between your Mac and iPad? I'm on Big Sur 11.6.1 ... wonder if your signature is up to date, Marc - are you really still on Mojave? Maybe there's an issue there? Try air dropping a Word DOC file (e.g.) and see if you're prompted to open Word on your iPad. (My iPad presents a list of 10 or more apps that can open .doc files in that case.)
One of the forum moderators • AC 25 USA and earlier • MacOS 11.6.5, iMac Pro