Error when trying to sort an expanded data set

Hello together,
when using orderby together with an odata expand operation I get and error. The statement I am trying looks as follows:

https://morpheus.fritz.box/odata4/svc/my_nutri_diary/Diary?$select=idDiaryEntry,MealNumber&$expand=fkDiaryToFDBProducts($select=brands,idCode,product_name)&$filter=AddedDateTime%20ge%202019-03-06T00:00:00%2B01:00%20and%20AddedDateTime%20le%202019-03-07T00:00:00%2B01:00&$orderby=MealNumber%20desc&$skip=0&$top=100

The error is:

<error xmlns="http://docs.oasis-open.org/odata/ns/metadata">
<code>null</code>
<message>
TEIID31202 Detected that an already sorted set of values was not in the expected order (typically UTF-16 / UCS-2). Please check the translator settings to ensure character columns used for joining are sorted as expected.
</message>
</error>

Please note, when using orderby without expand, the orderby is working as expected. The attribute which shall be used for ordering has the following type:

<Property Name="MealNumber" Type="Edm.String" Nullable="false" MaxLength="1">

In the underlaying database it is defined as an ENUM('1','2','3','4','5')