Currently, I could not reproduce your issue on my side.
Please check if you have chosen a proper instance (dataset) when you create a connection from PowerApps to your D365 Business Central data source.
Please also check if the std. webservice (tables) and your Purchase Order webservice are in same Business Central instance. If not, please re-create a new connection to your corresponding Business Central instance from your PowerApps.
More details about creating a connection from PowerApps to Business Central, please check the following video: