[Content Assist] does not work properly without org.eclipse.xtext.common.Terminals

Given is the following xtext grammar:

grammar org.xtext.example.mydsl.MyDsl

generate myDsl "http://www.xtext.org/example/mydsl/MyDsl"

PolygonBasedShape:
    shape = PolygonBasedNodeShape;

enum PolygonBasedNodeShape:
    octagon | oval |
    parallelogram | pentagon | plain | plaintext | point | polygon | primersite | promoter | proteasesite | proteinstab |
    rarrow | record | rect | rectangle | restrictionsite | ribosite | rnastab | rpromoter;

Use Case1 (Content Assistant works properly):

1. Create an empty test.mydsl file based on the grammar above and open it with the generated editor.
2. Press Ctrl+Space to invoke the Content Assistant.
3. Type the letter 'p' and 'r'.
   The Content Assistant suggests correct proposals beginning with the prefix 'pr'.
   

Use Case2 (Content Assistant does not work properly):

1. Create an empty test.mydsl file based on the grammar above and open it with the generated editor.
2. Type the letter 'p' and 'r'.
3. Press Ctrl+Space to invoke the Content Assistant.
   The Content Assistant suggests incorrect proposals beginning with the prefix 'r'.
   

HINT: When modifying the grammar to use the "org.eclipse.xtext.common.Terminals", the content assistant works for both of the use cases properly.

Reproduce the two content-assistant use cases with the MyDsl grammar shown, first without and then with org.eclipse.xtext.common.Terminals. Compare the proposals after typing “pr” before invoking Ctrl+Space versus invoking it afterward; done means both flows suggest entries matching the full prefix.