I exported the solution from the non-production environment that is experiencing the issue, and below is what I found in the customizations.xml file. It does appear that the 8 and the 2020 entries are there.
<attribute PhysicalName="EmailSender">
<Type>lookup</Type>
<Name>emailsender</Name>
<LogicalName>emailsender</LogicalName>
<RequiredLevel>none</RequiredLevel>
<DisplayMask>ValidForAdvancedFind|ValidForGrid</DisplayMask>
<ImeMode>auto</ImeMode>
<ValidForReadApi>1</ValidForReadApi>
<IsCustomField>0</IsCustomField>
<IsAuditEnabled>1</IsAuditEnabled>
<IsSecured>0</IsSecured>
<IntroducedVersion>6.1.0.0</IntroducedVersion>
<SourceType>0</SourceType>
<IsGlobalFilterEnabled>0</IsGlobalFilterEnabled>
<IsSortableEnabled>0</IsSortableEnabled>
<IsDataSourceSecret>0</IsDataSourceSecret>
<AutoNumberFormat></AutoNumberFormat>
<IsSearchable>0</IsSearchable>
<IsFilterable>0</IsFilterable>
<IsRetrievable>0</IsRetrievable>
<IsLocalizable>0</IsLocalizable>
<LookupStyle>multi</LookupStyle>
<LookupTypes>
<LookupType id="00000000-0000-0000-0000-000000000000">4</LookupType>
<LookupType id="00000000-0000-0000-0000-000000000000">4000</LookupType>
<LookupType id="00000000-0000-0000-0000-000000000000">2</LookupType>
<LookupType id="00000000-0000-0000-0000-000000000000">1</LookupType>
<LookupType id="00000000-0000-0000-0000-000000000000">8</LookupType>
<LookupType id="00000000-0000-0000-0000-000000000000">2020</LookupType>
</LookupTypes>