Hi Andy,
its exactly as David said.
The bug was unfortunately introduced when changing from R implementations to Python implementations of ppois (improving speed and removing dependancies). Thereby I forgot to change a few lines. I already send a pull request fixing the bug but as I am not maintainer of the repo I can't merge.
The fix is - as David said - to delete the 'lower_tail=False, log_bool = True' from the source code (in reads.py).
best ,
Simon