Entry created or updated - not getting all fields

I have a flow that fires on an entry being created or updated in a custom module in CRM. It is only supposed to proceed if a particular field is false and once fired, immediately updates that field to true to avoid running multiple times. For some reason, it was still duplicating runs.

I looked into the output from the initial trigger to see why it wasn't recognising that the value was true after the first run. Whilst the field exists on CRM and is available to update within Update Module in Flow, it does not seem to pull through in the initial trigger payload. It seems to pull through all other fields though. Is there a reason why certain fields don't show?