Other hashtags like #btc, #crypto, #ethereum are working fine. Only problem is with the Bitcoin hashtag.
I attempted with #Bitcoin and it turns out to be working fine for me. Possibly it's not an permanent issue as different hashtags like #BTC and #Crypto are working with you.
Facebook is likely out dated stuff with exceptionally less fascinating activities, its better unloading it from your list of online media. They officially don't support any crypto ads.