I am facing triple parsing errors triggered by annotations on XSD datatypes. Unfortunately, I was not able to come up with clear reproduction steps yet. The problem occurs in Protege as well as in ROBOT. Therefore, I think it is an OWL API problem.

If a datatype is annotated with some property (e.g xsd:decimal rdfs:isDefinedBy <http://example.org/my/ontology#> ., e.g. due to ontodev/robot#1163; also with other properties; property might be declared or undeclared -> not #1026), Protege sometimes shows the datatype in the class hierarchy too (see first image) or sometimes it is replaced in some restrictions with an Error resource (see second image) or sometimes both. However, the error does not occur always for the same file and if it occurs, not always for all restrictions that use the datatype. But an occurrence seems to become more likely for larger ontology files. Each occurrence comes together with an parsing error in the logs:

ERROR  09:25:56  Entity not properly recognized, missing triples in input? http://org.semanticweb.owlapi/error#Error1 for type Class
INFO  09:25:56  Unparsed triple: _:genid-nodeid-node1hg06utqmx78 -> http://www.w3.org/2002/07/owl#withRestrictions -> _:genid-nodeid-node1hg06utqmx79
INFO  09:25:56  Unparsed triple: _:genid-nodeid-node1hg06utqmx78 -> http://www.w3.org/2002/07/owl#onDatatype -> http://www.w3.org/2001/XMLSchema#decimal

But the triples in the TTL file are fine:

[…]
  owl:someValuesFrom [ rdf:type rdfs:Datatype ;
					   owl:onDatatype xsd:decimal ;
					   owl:withRestrictions ( [ xsd:maxInclusive 0.01
											  ]
											)
					 ]
[…]

The same error log entry is sometimes shown by ROBOT.