Note that this occurred when testing the FwData API and using the FwData API for round-tripping.
I assume it has something to do with Morph Types.

Error: MiniLcm.Exceptions.CreateObjectException : Failed to create entry Entry { Id = f1d74029-2323-580f-00e2-96d1867f097b, DeletedAt = , LexemeForm = fr: cross-media, en: Lane, CitationForm = fr: transmit, es: input, en: forecast, LiteralMeaning = fr: MiniLcm.Models.RichString, es: MiniLcm.Models.RichString, MorphType = Enclitic, Senses = System.Collections.Generic.List1[MiniLcm.Models.Sense], Note = fr: MiniLcm.Models.RichString, en: MiniLcm.Models.RichString, Components = System.Collections.Generic.List1[MiniLcm.Models.ComplexFormComponent], ComplexForms = System.Collections.Generic.List1[MiniLcm.Models.ComplexFormComponent], ComplexFormTypes = System.Collections.Generic.List1[MiniLcm.Models.ComplexFormType], PublishIn = System.Collections.Generic.List`1[MiniLcm.Models.Publication] }
---- SIL.LCModel.LcmInvalidFieldException : Fieldname 'Affixes' does not exist. Consider using a decorator or review the fieldname for typos.

Failed FwLiteProjectSync.Tests.CrdtEntrySyncTests.CanSyncRandomEntries(roundTripApiType: FwData) [2 s]
Error Message:
MiniLcm.Exceptions.CreateObjectException : Failed to create entry Entry { Id = f1d74029-2323-580f-00e2-96d1867f097b, DeletedAt = , LexemeForm = fr: cross-media, en: Lane, CitationForm = fr: transmit, es: input, en: forecast, LiteralMeaning = fr: MiniLcm.Models.RichString, es: MiniLcm.Models.RichString, MorphType = Enclitic, Senses = System.Collections.Generic.List1[MiniLcm.Models.Sense], Note = fr: MiniLcm.Models.RichString, en: MiniLcm.Models.RichString, Components = System.Collections.Generic.List1[MiniLcm.Models.ComplexFormComponent], ComplexForms = System.Collections.Generic.List1[MiniLcm.Models.ComplexFormComponent], ComplexFormTypes = System.Collections.Generic.List1[MiniLcm.Models.ComplexFormType], PublishIn = System.Collections.Generic.List1[MiniLcm.Models.Publication] } ---- SIL.LCModel.LcmInvalidFieldException : Fieldname 'Affixes' does not exist. Consider using a decorator or review the fieldname for typos. Stack Trace: at FwDataMiniLcmBridge.Api.FwDataMiniLcmApi.CreateEntry(Entry entry, CreateEntryOptions options) in D:\a\languageforge-lexbox\languageforge-lexbox\backend\FwLite\FwDataMiniLcmBridge\Api\FwDataMiniLcmApi.cs:line 1009 at FwLiteProjectSync.Tests.EntrySyncTestsBase.CanSyncRandomEntries(Nullable1 roundTripApiType) in D:\a\languageforge-lexbox\languageforge-lexbox\backend\FwLite\FwLiteProjectSync.Tests\EntrySyncTests.cs:line 194
--- End of stack trace from previous location ---
----- Inner Stack Trace -----
at SIL.LCModel.Infrastructure.Impl.LcmMetaDataCache.GetFieldId2(Int32 clid, String fieldName, Boolean includeBaseClasses)
at SIL.LCModel.DomainServices.VirtualOrderingServices.CheckParentAndGetFieldName(ICmObject parent, Int32 flid)
at SIL.LCModel.DomainServices.VirtualOrderingServices.GetOrderedValue(ICmObject parent, Int32 flid, IEnumerable1 unmodifiedSeq) at SIL.LCModel.DomainServices.VirtualOrderingServices.GetOrderedValue[T](ICmObject parent, Int32 flid, IEnumerable1 unmodifiedSeq)
at SIL.LCModel.DomainImpl.LexEntry.get_VisibleComplexFormBackRefs()
at SIL.LCModel.DomainImpl.LexEntryRef.UpdateComplexFormEntryBackRefs(ICmObject thingAddedOrRemoved, Boolean fAdded)
at SIL.LCModel.DomainImpl.LexEntryRef.AddObjectSideEffectsInternal(AddObjectEventArgs e)
at SIL.LCModel.DomainImpl.CmObject.SIL.LCModel.ICmObjectInternal.AddObjectSideEffects(AddObjectEventArgs e)
at SIL.LCModel.DomainImpl.LcmList1.AddObject(AddObjectEventArgs args) at SIL.LCModel.DomainImpl.LcmReferenceSequence1.AddObject(AddObjectEventArgs args)
at SIL.LCModel.DomainImpl.LcmList1.Add(T obj) at SIL.LCModel.DomainImpl.LexEntry.AddComponent(ICmObject other) at FwDataMiniLcmBridge.Api.FwDataMiniLcmApi.InsertComplexFormComponent(ILexEntry lexComplexForm, ICmObject lexComponent, BetweenPosition1 between) in D:\a\languageforge-lexbox\languageforge-lexbox\backend\FwLite\FwDataMiniLcmBridge\Api\FwDataMiniLcmApi.cs:line 1112
at FwDataMiniLcmBridge.Api.FwDataMiniLcmApi.AddComplexFormComponent(ILexEntry lexComplexForm, ComplexFormComponent component, BetweenPosition`1 between) in D:\a\languageforge-lexbox\languageforge-lexbox\backend\FwLite\FwDataMiniLcmBridge\Api\FwDataMiniLcmApi.cs:line 1096
at FwDataMiniLcmBridge.Api.FwDataMiniLcmApi.<>c__DisplayClass125_0.b__0() in D:\a\languageforge-lexbox\languageforge-lexbox\backend\FwLite\FwDataMiniLcmBridge\Api\FwDataMiniLcmApi.cs:line 996
at SIL.LCModel.Infrastructure.UndoableUnitOfWorkHelper.Do(String undoText, String redoText, IActionHandler actionHandler, Action task)
at SIL.LCModel.Infrastructure.UndoableUnitOfWorkHelper.DoUsingNewOrCurrentUOW(String undoText, String redoText, IActionHandler actionHandler, Action task)
at FwDataMiniLcmBridge.Api.FwDataMiniLcmApi.CreateEntry(Entry entry, CreateEntryOptions options) in D:\a\languageforge-lexbox\languageforge-lexbox\backend\FwLite\FwDataMiniLcmBridge\Api\FwDataMiniLcmApi.cs:line 964
[xUnit.net 00:00:34.39] FwDataMiniLcmBridge.Tests.MiniLcmTests.WritingSystemTests.GetWritingSystems_ReturnsExemplars [SKIP]