Previously the default was long, which is 32-bit on AVR. But avr-gcc expects a smaller value: it reads the return value from r24.
This is actually a regression from https://reviews.llvm.org/D98205. Before D98205, the return value was an enum (which was 2 bytes in size) which was compatible with the 1-byte return value that avr-gcc was expecting. But long is 4 bytes and thus places the significant return value in a different register.