Fake documents were circulated by the pro-Bush side, the folks at CBS and Dan Rather were either really stupid for falling for it or just doing what fake news does.
The truthful story concerning Bush using the Guard to avoid Vietnam and his refusal to honor his Guard service commitments without penalty was buried after a successful
disinformation op.
Bush was and always will be a tool of the Deep State.
There is no way for me to know for certain why Bush refused to submit to a required flight physical, but the most likely explanation was his cocaine use at the time had him worried about a drug test.
Keep in mind the tax payers had spent over a million dollars training Bush to be a pilot, and then he refused to get a flight physical. Anyone else would have been disciplined under the UCMJ.
Thousands of good men were on the list for that slot who would have honored their commitment, but were passed over so low scoring Bush could avoid Vietnam .
Thanks to Dan Rather, no one demanded a explanation as to how a lieutenant could refuse a required flight physical and avoid discipline.