I know this is a strange problem but just wondered if anybody had any
ideas as to why this should happen.

A user received a meeting request. They accepted it.

The email stayed in their inbox rather than moving to their Deleted
Items and did not show up in their calendar. When they clicked the
appointment in their inbox it showed as accepted at the top of the
email.

A few hours later they went back into the email which said that it had
NOT been accepted. They then had to re-accept this - the second time
it went into his deleted items and showed up in his calendar.

Seems strange doesn't it?