I'm afraid this is a known issue in 11.1 - at least it's been reported by a few users. I'm not sure what the underlying factors are - is your model reading from a database? Are there compund filters? Is there a sort order? Are there external lookups? Do any subsequent definitions (calculated fields, sorts, filters, joins) depend on fields from an external lookup?
Two options - upgrade to v12, and I've certainly not heard of these problems beyond 11.5 - or break down the model. One problem - v11.5 or above prohibits using batch files unless you pay for an special licence.
By "breaking down the model" I mean creating a new model for each possible step that's introducing the error. So the first model would be to open the data source, define the fields, and export to a local Access database. The second might be to filter this, and export again to a new local Access database. The third might be to bring in an external lookup, and again, create a local Access database...
Sorry not to have better news,
Many thanks for coming back so soon. I decided yesterday to rebuild the model from scratch (it was originally built several years ago). The export has been tested several times using the new model and the issue seems to have een resolved. Thanks for the information below and we will take this on board for any future issues of this kind. To be fair you use Monarch very intensively in my company and this is only the second time I have seen this issue in 13 years.