Why would it be? B450 is literaly B350 with different firmware. It's the same silicon. So if a board design just swapped chipsets and didn't do anything else, you could theoretically easily crossflash without issue.
Yeah, asrock had plenty of quirky boards in their past.
Like for example 945 series with core2 duo support https://www.asrock.com/mb/Intel/ConRoe1333-DVIH/index.asp#BIOS
Or 1156 with P67 (normally a 1155 chipset) https://www.asrock.com/mb/Intel/P67%20Transformer/index.asp
Or 775 with Core2 Duo...