I got to thinking about the oldest version of Cubase I have access to, and, although the Atari ST is still out in the woodshed, I wasn’t sure I could get it working again and didn’t have the old dongle either. Luckily a lockdown attic clearout had revealed a box with my earliest Windows version, namely Cubasis VST 5, which dates back to the days when Steinberg was owned by Pinnacle. I also had an old laptop with Windows XP and the rest was easy, I loaded your .arr and the tracks appeared, saved it as .all, imported it with SX3 and saved it as .cpr which then loaded into Cubase 10.5.20
So somewhere between the “VST 5” and “SX” generations, Cubase lost a compatibility with .arr files from the Atari era.