JQL filter with only numeric "assignee" field doesn't work with WAS operator

Issue Summary

If the lower_user_name is changed to only numeric value, JQL query with WAS operator doesn't return expected results

Environment

7.13.6

Steps to Reproduce

1. Create a user with numeric only username or Rename a user from text to numeric viz., abrajtxhd to 45671234
2. Assign some issue(s) to 45671234
3. Run assignee was 43387518

select * from app_user where lower_user_name like '%45671234%' or user_key like '%45671234%';

Expected Results

JQL filter returns current and old issues with assignee 43387518

Actual Results

JQL filter returns no result:

Workaround

The workaround is to use double-quote or single quote around the numeric username to get the expected results:

assignee was '45671234'