SpringBoot/MongoDB returns error code 251 - spring

Question

We use Spring Boot 3.0.2 (which in turn uses the mongo driver 4.8.2). We use MongoDB 6.0.4, replica set size 1.
We use the reactive stack and the transactional operator to demarcate the transactions like this:
Mono.just(initialState).flatMap(queryDatabase).flatMap(createMongoDocument1InCollectionA).flatMap(updateMongoDocument2InCollectionB).as(transactionalOperator::transactional).retryWhen(retrySpec)
We use read and write concern majority (even if it is not relevant with a replica set size 1). All other settings, e.g. session synchronization, are default.
If we run this code in parallel in multiple threads it often (about 20%) fails with the following message:
Command failed with error 251 (NoSuchTransaction): 'Given transaction number 8 does not match any in-progress transactions. The active transaction number is 7' on server localhost:49633. The full response is {"errorLabels": ["TransientTransactionError"], "ok": 0.0, "errmsg": "Given transaction number 8 does not match any in-progress transactions. The active transaction number is 7", "code": 251, "codeName": "NoSuchTransaction", "$clusterTime": {"clusterTime": {"$timestamp": {"t": 1675160699, "i": 11}}, "signature": {"hash": {"$binary": {"base64": "75m2LsbMsyLuFwtSHZPInaFs4Lo=", "subType": "00"}}, "keyId": 7194760344735055879}}, "operationTime": {"$timestamp": {"t": 1675160699, "i": 11}}}
The retry normally helps here, but not always (in case the max attempts count is exceeded). We did not observe this error if we have just one thread executing a single request. We also noticed, that if multiple requests from the parallel threads do not try to modify the same Mongo document, this error occurs much less often (maybe about 5%).
We observed the same error occuring even more often (about 60%) with Spring Boot 2.7.6 (mongo driver 4.6.1) and MongoDB 6.0.4 (or MongoDB 4.2.0). The error message was:
Query failed with error code 251 and error message 'Given transaction number 1 does not match any in-progress transactions. The active transaction number is -1' on server localhost:49372;
The issue will certainly cause a significant operation disruption in the high concurrency production environment.
Any help on the explanation and the fix of the issue would be very appreciated.