Unexplainable SQL behaviour - sql

Question

I have a simple SQL query, with a handful of joined queries.
In my select I have a function shaping the date to a string format as below
LEFT(REPLACE(ADM_DT,'-',''),8)+REPLACE(ADM_TM,':','')+'00'
The script I am running should return 84 rows currently, but when executing the above in the select, it continuosly returns a different volume of rows every execution? there are no changes to the underlying tables between executions.
The strange thing is that whe I swith this back to a standard date conversion, the rows total 84
CONVERT(DATETIME,CONVERT(NVARCHAR,ADM_DT)+' '+ADM_TM)
Can anyone explain why SQL is behaving this way, I have not seen this before?
Full script below:
SELECT
[Visit ID] = a050.HSP_NO
,[Current Site] = currwd.HOSP
,[Current Specialty] = curr.SPEC
,[Current Ward] = currwd.WARD
,[Current Ward Name] = REPLACE(currwd.CURRDESC,'"','')
,[Current Consultant] = curr.PROF
,[Admission Date] = LEFT(REPLACE(ADM_DT,'-',''),8)+REPLACE(ADM_TM,':','')+'00'
,[Active] = '1'
,[PAS ID] = [crn].[crn]
,[Patient Class] = 'IP'
,NHS.[nhsno]
,CHI.CHI
,CASE WHEN OSV.X_CN IS NOT NULL THEN 'OSV' ELSE '' END OSV
,CASE WHEN NHS.[nhsno] LIKE '7%' THEN '7 NHSNO' ELSE '' END [PROBLEM NHS NO]
,DATEDIFF(D,PATS.DATE_OF_BIRTH,CONVERT(DATE,ADM_DT)) /365 [AGE ON ADMISSION]
FROM PCSSSA..SILVER.APK050_HPROVSPELL a050 WITH (NOLOCK)
-- Current Admission details
LEFT JOIN (
SELECT X_CN, CEP_NO, HSP_NO, SPEC, PROF --MSPEC
FROM PCSSSA..SILVER.APK051_CONEPIS epi WITH (NOLOCK)
-- Main Specialty Map
LEFT JOIN PCSSSA..SILVER.ENV050_DISCIPDETS en050 WITH (NOLOCK)
ON epi.SPEC = en050.OBJ_DISC
AND en050.OBJ_TYPE='SP'
AND en050.DATE_TO IS NULL
)curr
ON curr.X_CN = a050.X_CN
AND a050.HSP_NO = curr.HSP_NO
AND curr.CEP_NO = (SELECT TOP 1 a051.CEP_NO
FROM PCSSSA..SILVER.APK051_CONEPIS a051 WITH (NOLOCK)
Where a051.X_CN = a050.X_CN AND a050.HSP_NO = a051.HSP_NO
Order By CEP_NO DESC)
-- Current Ward Detail
JOIN (SELECT *, ROW_NUMBER() OVER (PARTITION BY X_CN, CEP_NO ORDER BY WS_NO DESC) AS WR
FROM PCSSSA..SILVER.APK052_WARDSTAY wdstay WITH (NOLOCK)
LEFT JOIN (SELECT
CURRWARD = OBJ_LOC,
CURRDESC = OBJ_DESC
FROM [PCSSSA]..[SILVER].[ENV030_LOCDETS] WITH (NOLOCK)
WHERE OBJ_TYPE = 'WARD'
AND DATE_TO IS NULL
)wdname
ON wdstay.WARD = wdname.CURRWARD
) currwd
ON curr.X_CN = currwd.X_CN
AND curr.CEP_NO = currwd.CEP_NO
AND currwd.WR=1
--- Admitting details
LEFT JOIN (
SELECT X_CN, CEP_NO, HSP_NO, PROF, SPEC --MSPEC
FROM PCSSSA..SILVER.APK051_CONEPIS epi WITH (NOLOCK)
-- Main Specialty Map
LEFT JOIN PCSSSA..SILVER.ENV050_DISCIPDETS en050 WITH (NOLOCK)
ON epi.SPEC = en050.OBJ_DISC
AND en050.OBJ_TYPE='SP'
AND en050.DATE_TO IS NULL
)adm
ON adm.X_CN = a050.X_CN AND a050.HSP_NO = adm.HSP_NO
AND adm.CEP_NO = (SELECT TOP 1 a051.CEP_NO
FROM PCSSSA..SILVER.APK051_CONEPIS a051 WITH (NOLOCK)
Where a051.X_CN = a050.X_CN AND a050.HSP_NO = a051.HSP_NO
Order By CEP_NO)
-- Admitting Ward Detail
JOIN (SELECT *, ROW_NUMBER() OVER (PARTITION BY X_CN,CEP_NO ORDER BY WS_NO) AS WR FROM PCSSSA..SILVER.APK052_WARDSTAY WITH (NOLOCK)) admwd
ON adm.X_CN = admwd.X_CN
AND adm.CEP_NO = admwd.CEP_NO
AND admwd.WR=1
-- Patient Detail
LEFT JOIN
(SELECT
[id].[RM_PATIENT_NO],
[id].[NUM_ID_TYPE] + CONVERT(NVARCHAR,[NUMBER_ID]) [crn]
FROM [PCSSSA]..[SILVER].[NUMBER_IDS] [id] WITH (NOLOCK)
WHERE [id].[NUM_ID_TYPE] IN ('0', '1', 'W')
)[crn]
ON a050.[X_CN] = [crn].[RM_PATIENT_NO]
-- NHS NUMBERS
LEFT JOIN
(
SELECT
[id].[RM_PATIENT_NO],
[id].[NUMBER_ID] [nhsno]
FROM [PCSSSA]..[SILVER].[NUMBER_IDS] [id] WITH (NOLOCK)
WHERE [id].[NUM_ID_TYPE] = ('NHS')
)NHS
ON NHS.RM_PATIENT_NO = a050.X_CN
-- CHI NUMBER
LEFT JOIN
(
SELECT
[id].[RM_PATIENT_NO],
[id].[NUMBER_ID] [CHI]
FROM [PCSSSA]..[SILVER].[NUMBER_IDS] [id] WITH (NOLOCK)
WHERE [id].[NUM_ID_TYPE] IN ('CHI')
)CHI
ON CHI.RM_PATIENT_NO = a050.X_CN
-- OVERSEES STATUS
LEFT JOIN
(
SELECT X_CN, [STATUS], SDATE, EDATE FROM PCSSSA..SILVER.CRS037_OSV_STATUS WITH (NOLOCK)
)OSV
ON OSV.X_CN = a050.X_CN
AND CONVERT(DATE,ADM_DT) >= OSV.SDATE
AND (OSV.SDATE IS NULL
OR CONVERT(DATE,ADM_DT) <= OSV.EDATE)
-- DEMOGRAPHICS
LEFT JOIN
(
SELECT RM_PATIENT_NO, DATE_OF_BIRTH FROM PCSSSA..[SILVER].[PATIENTS] WITH (NOLOCK)
)PATS
ON PATS.RM_PATIENT_NO = a050.X_CN
-- CURRENTLY ADMITTED ONLY
WHERE DIS_DT IS NULL
-- WITHOUT NHS NUMBER
AND (nhsno IS NULL
-- OR BRING IN ANY 7 NHS NUMBERS FOR CORRECTION
OR NHS.[nhsno] LIKE '7%'
-- ALSO INCLUDE OVERSEES
OR OSV.X_CN IS NOT NULL)
What is even stranger, is that if you wrap this in a subquery and coun(*) on the outer then it totals 84 as expected, very strange!