Look-up field values disappear on a child table

The lookup values disappear on the child table after updating a formula text field on the child table.

Summary

Step 1 - Add a formula text field on "Checkpoint" table.  This formula will set the value of the "Checkpoint Status" field on the Checkpoint table:    No issues with this step, values are set correctly

 Step 2 - Set up a relationship between Checkpoint (parent) and Sub-task (child) tables.  Add the "Checkpoint Status" field to the Sub-task table:  No issues - values from the parent Checkpoint status field are correctly displayed on the child "Sub-tasks" table

 Step 3 - Update the formula text field "Status"  on the Sub-task table to check for a value in the "Checkpoint Status" field

 .....

[Checkpoint - Chk Status] =  "Cancelled" and IsNull([Actual Start - Auto]) and IsNull([Actual End - Auto]),

[Checkpoint - Chk Status],

 .....

 Issue:  Once this logic was added ALL the values that were previously displayed on the Sub-tasks form (see step 2) from the "Checkpoint Status" lookup field disappeared.

Once I commented out the logic then there was no issue.

 

I have tried the following

(1) 'Commented out' the "IsNull ..... Logic and only checked for the value "Cancelled", however this did not work

(2) Deleted the columns, created the relationship ..... Same results

 

Anyone having this problem?