So... I tried to follow your code as a sample.
This is what I had before:
//Creates a collection from the table to be exported
ClearCollect(
ExportTable,
Search(
AddColumns(
Filter('[dbo].[JourneyUser]',
(BatchNameDropdown.Selected.Result = "All" || BatchName = BatchNameDropdown.Selected.Result)
),
"E911", LookUp(UserResponse, JourneyUserID=id && QuestionID=34, Response)
),
TextInput2.Text, "BatchName"
)
);
//Creates variable that eliminates extra columns from the table
Set(varDroppedColumns,
DropColumns(
ExportTable,
"CurrentPhase", "CreatedDatetime", "CurrentPhaseStartDate", "id", "JourneyActualStartDate", "JourneyPlannedStartDate", "MigrationCompletedDateTime", "Phase1EndDate", "Phase1PlannedStartDate", "Phase2PEndDate", "Phase2PlannedStartDate", "Phase3EndDate", "Phase3PlannedStartDate", "UpdatedDatetime", "CreatedBy", "MigrationComplete", "MigrationCompletedByAdmin", "MigrationCompleteEmailSent", "TenantID", "UpdatedBy", "JourneyPathID", "isInActive"
)
);
//Sets variable that formats the JSON File
Set(varFormattedJSON,
JSON(varDroppedColumns, JSONFormat.IndentFour)
);
//Executes the flow
E911SendMail.Run(varFormattedJSON);
//Navigates to Sent screen
Navigate(SuccessScreen, ScreenTransition.Fade)
This is what I previewed (which is the right fields, except for the fact that they're empty except for the first column):

Then, I modified the code as follows:
/Creates a collection from the table to be exported
ClearCollect(
ExportTable,
DropColumns(
Search(
AddColumns(
Filter('[dbo].[JourneyUser]',
(BatchNameDropdown.Selected.Result = "All" || BatchName = BatchNameDropdown.Selected.Result)
),
"E911", LookUp(UserResponse, JourneyUserID=id && QuestionID=34, Response)
),
TextInput2.Text, "BatchName"
),
"CurrentPhase",
"CreatedDatetime",
"CurrentPhaseStartDate",
"id",
"JourneyActualStartDate",
"MigrationCompletedDateTime",
"Phase1EndDate",
"Phase1PlannedStartDate",
"Phase2PEndDate",
"Phase2PlannedStartDate",
"Phase3EndDate",
"Phase3PlannedStartDate",
"UpdatedDatetime",
"CreatedBy",
"MigrationComplete",
"MigrationCompletedByAdmin",
"MigrationCompleteEmailSent",
"TenantID",
"UpdatedBy",
"JourneyPathID",
"isInActive"
)
);
//Creates variable that eliminates extra columns from the table
Set(varDroppedColumns,
ExportTable
);
//Sets variable that formats the JSON File
Set(varFormattedJSON,
JSON(varDroppedColumns, JSONFormat.IndentFour)
);
//Executes the flow
E911SendMail.Run(varFormattedJSON);
//Navigates to Sent screen
Navigate(SuccessScreen, ScreenTransition.Fade)
And now it doesn't find any data at all...

In both cases, the flow seems to work well: when I execute the code, I do receive an email with an Excel file attached (which is the intended purpose of the app), but I only see two fields...

Any suggestions on how to fix this?