Another option if you don't want to completely disable bitlocker is to simply create a new partition from Windows to allocate the space, then delete it. This will leave an unallocated section which you can use to create the non bitlocked ext4 partition from the iso installer
BitLocker would still ask for recovery key when attempting to boot Windows after Arch install is finished. This is because of the disk configuration change that it detects.
I noticed that as well but in my case it seemed to be due to secure boot being turned off. Annoying nonetheless!