When I run ff_transactions() on this league:
mfl_connect(season = 2017, league_id = 22686)

I get this warning message:
Warning message:
Problem with mutate() input bid_amount.
i NAs introduced by coercion
i Input bid_amount is as.numeric(.data$bid_amount).

I know we used the MFL feature of bids being presented in millions in that season. It seems like something in ff_transactions() is struggling with that format of bids. It has the same issue with 2016 for that league ID.